On the first anniversary of Bnai Yisrael’s travels exodus from Egypt the nation celebrated the festival of Pesach. This observance included the offering of the Pesach sacrifice. This sacrifice cannot be offered on behalf of those that are defiled. A group of individuals had come into contact with a dead body. As a result, they were excluded from the sacrifice.
These individuals regretted that they could not participate in the Pesach sacrifice. They explained their concerns to Moshe.
Moshe approached Hashem. The Almighty responded that there is a solution for these people. Hashem revealed the mitzvah of Pesach Sheynee. This mitzvah provides a second opportunity to participate in the Pesach. If a person cannot participate in the Pesach on the 14th of Nisan, this individual offers the Pesach one month latter.
This opportunity is afforded to a variety of individuals. Included are those that are not able to participate in the offering of the Pesach on the 14th of Nisan. In the wilderness, this group primarily consisted of individuals that were defiled. After the conquest of the land of Israel, this group also included those that could not reach the Temple by the 14th of Nisan. Pesach Sheynee also applies to a person that do not participate in the offering of the 14th of Nisan out of negligence. This person is also obligated to participate in Pesach Sheynee.
There is an amazing discussion in Tractate Pesachim regarding the mitzvah of Pesach Sheynee. The discussion revolves around an interesting question. After the sacrifice, the meat of the Pesach is consumed by those bringing the offering. The Pesach is not eaten alone. It must be consumed with matzah and marror. Assume a person is defiled or unable to reach the Temple on the 14th of Nisan. This person’s condition prevents participating in the Pesach offering on this date. However, nothing prevents this person from eating the matzah an marror that accompanies the Pesach. These can be consumed by a defiled person. They can be eaten outside of area of the Temple. Does this person consume the matzah and marror, without the Pesach?
The Talmud responds that the person does eat matzah and marror on the 14th of Nisan. Inability to participate in the Pesach offering does not interfere with the obligation to eat matzah and marror on the 14th of Nisan. To this point, there is nothing odd in the Talmud’s discussion.
Next, the Talmud discusses the source for this law. Where does the Torah tell us that it is required to eat matzah and marror? The Talmud offers two responses. We will concentrate on one response. The Talmud suggests that the source is the pasuk that states, “In the evening you shall eat matzah.” The Talmud explains that this passage teaches us that the obligation to eat matzah and marror apply even in a case in which the Pesach cannot be consumed.
This brings us to the remarkable portion of the Talmud’s discussion. The Talmud explains that the message of this pasuk is not at all obvious. One might think that some individuals would not eat matzah and marror on the 14th of Nisan. One could reason that those obligated in Pesach Sheynee do not eat matzah and marror on the 14th of Nisan. Instead, they fulfill these obligations in conjunction with the mitzvah of Pesach Sheynee. Therefore, the pasuk stating, “In the evening you shall eat matzah” is included in the Torah. The passage indicates that everyone eats matzah and marror on the 14th of Nisan. The passage specifically includes those that will participate in Pesach Sheynee.
This discussion presents a major problem. The passage stating, “In the evening you shall eat matzah” was revealed to Moshe in Egypt. According to the Talmud, it specifically addresses the person that will celebrate Pesach Sheynee. In other words, the passage deals with a possible misunderstanding created by the mitzvah of Pesach Sheynee. This implies that if the mitzvah of Pesach Sheynee did not exist, it would be self-evident that everyone eats matzah and marror on the 14th of Nisan and the passage would not be needed. This does not seem to make sense. In fact, the mitzvah of Pesach Sheynee did not exist in Egypt. It only was revealed latter, in the wilderness! The Torah is responding to a mitzvah that had not yet been revealed!
There is an interesting comment by Rashi that indirectly deals with this issue. Rashi begins with an interesting question. The mitzvah of Pesach Sheynee was revealed in response to an inquiry. Defiled individuals approached Moshe seeking an opportunity to participate in the Pesach sacrifice. Why did the Almighty not reveal the commandment of Pesach Sheynee at Sinai? Why did He wait until these individuals approached Moshe? Rashi responds that the mitzvah could have been revealed at Sinai. However, the Almighty decided to delay communication of this mitzvah. He wished to reward these sincere individuals. He rewarded them through revealing a section of the Torah in response to their inquiry.
It seems, from Rashi’s comments, that this section was not an afterthought. It was part of the Torah revealed at Sinai. However, this specific commandment was withheld. This was done in order to communicate the mitzvah at the appropriate time. In other words, the Torah is an integrated whole. Each mitzvah is recorded as one of the 613 integrated mitzvot. Each mitzvah is consistent with all of the others.
This answers our question. It is true that the passage stating, “In the evening you shall eat matzah” deals with a problem created by Pesach Sheynee. It is also true that this passage was revealed before the mitzvah of Pesach Sheynee was given. However, the passage is designed as part of a complete Torah. The complete Torah includes all 613 mitzvot. Pesach Sheynee is one of these mitzvot. Therefore, it is reasonable for the mitzvah of matzah and marror to be recorded in a manner that is consistent with the command of Pesach Sheynee. The passage assumes the existence of Pesach Sheynee, even though the mitzvah has not yet been revealed.
“And when the ark went forth Moshe said, "Arise, Hashem and disperse Your enemies. And those that hate You will flee from before You". And when it came to rest he said, "Return, Hashem to the myriads of the thousand of Israel". (BeMidbar 12:35-36)
In a Torah scroll these two pesukim are set apart from the preceding and following passages. An inverted Hebrew letter is appears before the passages. The same inverted letter follows the passages. The letter used is the nun.
Why are these passages set apart? The Talmud, in Tractate Shabbat, explains that these passages are regarded as a separate book of the Torah. They are set apart to indicate this special status.
This only raises an additional question. Why are these passages given the status of a separate book of the Torah? There are various responses to this issue. One obvious approach is to attribute some special significance to the content of the passages. However, Rav Naftali Tzvi Yehudah Berlin Ztl (Netziv) offers another approach.
Netziv bases his explanation upon a discussion in Tractate Shabbat. The Talmud comments that the Torah is not actually composed of five books. It is composed of seven. Beresheit, Shemot VaYikra and Devarim are each counted as a single book. However, BeMidbar must be counted as three books. This is because our pesukim compose a separate book. This book divides BeMidbar into two additional books. The section preceding our passages is one book. Our pesukim constitute a second book. The section following our passages is a third book.
Netziv explains that our passages are not set apart because of their unique content. They are set apart in order to divide Sefer BeMidbar into two separate parts. This break is designed to contrast the first portion of the sefer with the material that will follow. What is this contrast?
The first part of the sefer depicts the close relationship between the Almighty and His nation. The sefer begins with a detailed description of the encampment in the wilderness. The various tribes camped around the Tabernacle. The influence of Hashem was manifest in the Mishcan. The Almighty was among the people. The inauguration of the Mishcan is described.
The mitzvah of sotah that is related in the pervious parasha captures this relationship. This test of a suspected adulterous relies on the intervention of the Almighty. The woman is given a mixture to drink. This drought is harmless. However, if the woman is guilty of adultery, Hashem will perform a miracle. The mixture will kill the woman. This entire concept assumes a remarkably close relationship between G-d and Bnai Yisrael.
The later section of the sefer depicts a different relationship. The nation begins to complain against the Almighty. They send spies to study the land of Israel. The nations refuses to enter the land. Korach and his followers rebel. As the nation removes itself from the Almighty, He responds. He distances Himself from His people. He condemns the generation that left Egypt to death in the wilderness. The ultimate exile of the nation from the land of Israel is decreed. Various other punishments are depicted, throughout the later half of the sefer.
We can now define the contrast contained in Sefer BeMidbar. The nation entered the wilderness with a unique closeness to the Almighty. The sefer contrasts this intimacy with the more distant relationship that developed in the course of the sojourn in the wilderness. Our pesukim are the dividing point between these two relationships.
We can now understand the reason Sefer BeMidbar is characterized as a single book and as three separate books. It can be described as three books because our pesukim divide the first portion of the sefer from the later portion. These two portions tell of very different relationships between the Almighty and His nation. Each relationship can be appropriately described as a separate stage of the development of Bnai Yisrael. On this basis the sections can be regarded as separate books divided by a third intervening book.
BeMidbar can also be described as a single book. It is designed to express contrast. The contrast is created through including the two relationships in a single book. From this perspective, BeMidbar deserves to be regarded as a single book. 
 Sefer Shemot 12:18.
 Mesechet Pesachim 120a with commentary of Rashbam.
 Rabbaynu Shlomo ben Yitzchak (Rashi), Commentary on Sefer BeMidbar 9:7.
 Mesechet Shabbat 116a.
 Mesechet Shabbat 116a.
 Rav Naftali Tzvi Yehuda Berlin (Netziv), Commentary Hamek Davar on Sefer BeMidbar, Introduction.