Our Sages have referred to the Bible as the "Five Books of the Torah" (See Rebbi Yitzchak, Megillah, chap.1, "Megillah Nikreis", pg. 15a; Rav Ada and Rebbi Chaninah, Nedarim, chap. 3, "Arba'a Nedarim", pg. 22b; Rav Yehudah bar Masparta, Sanhedrin, chap. 6, "Nigmar Hadin", pg. 44a.) This is unlike Rav Shmuel bar Nachman, (Shabbos, chap. 16, "Kol Kisvei", pgs. 115b-116a), who claims that there are seven Books of the Torah (Pro. 9:1). In this article, we will be focusing on the opinions that maintain that there are five books of the Torah.
Hashem looked into these five books and created the world from them (Zohar, Pinchas, pg. 216b). Moreover, the continued existence of the world is dependent on the acceptance to fulfil the Five Books of the Torah (ReishLakish, Shabbos, chap. 9, "Amar Rebbi Akiva", pg. 88a).
Rebbi Simon (Bereishis Rabba, 1:3) points out that the Five Books of the Torah correspond to the five times the word "light" is mentioned in the story of creation (Gn. 1:3-5). "Let there be light" (Gn. 1:3) corresponds to the Book of Genesis which discusses the light of creation. "And there was light" (Gn. 1:3) parallels to the Book of Shemos which talks about the light of redemption. "And God saw that the light was good" (Gn. 1:4) links to the Book of Vayikra which contains the light of many halachos (laws). "And God separated between the light and the darkness" (Gn. 1:4) matches to the Book of Bamidbar which speaks about the light of Eretz Yisrael. "And God called to the light: Day" (Gn. 1:5) relates to the Book of Devarim which encompasses the light of many halachos.
The Shvilei Pinchas says that whenever we study the Five Books of the Torah, we bring these five different types of light into the world. It is only then that we can go through the journey of life by the light of the Torah. Otherwise, we are likened to a blind person who gropes around in the darkness, never arriving at the desired destination.
The Shvilei Pinchas makes an observation. Of all the times that the aforementioned verses stated the word "light", only regarding the third "light" does it say that it was "good." By extension, this would mean that out of all Five Books of the Torah, only the third book, Vayikra, is called "good." This begs us to ask, "What is so "good" about Sefer Vayikra and its light that it is greater than the light contained in any of the other four sefarim?"
One approach in addressing this question is based on a Rashi (citing the Toras Kohanim, Lv. 19:1) that states that the majority of the Torah's essentials are contained in Sefer Vayikra. This is because of one verse in Sefer Vayikra which says, "And you must love your fellow as yourself" (Lv. 19:18). Hillel said that this commandment is the entirety of the Torah and the rest of the Torah is just a commentary to this principle (Shabbos, chap. 2, "Bameh Madlikin", pg. 31a).
After establishing that Sefer Vayikra contains the majority of Torah, we must add that the Torah itself is called "good" (Pirkei Avos, chap. 6, "Kinyan Torah", Beraisa 6, Pro. 4:2). Therefore, since Sefer Vayikra contains the majority of Torah, and Torah is called "good", it is not surprising that Sefer Vayikra and its "light" is called "good" (Shvilei Pinchas).
Perhaps we could add that there is another component which is unique to Sefer Vayikra. That is, Sefer Vayikra is known to be the book about the offerings. When you put this aspect together with the previous one, it turns out that Sefer Vayikra has two aspects to it which stand out; the directive about the offerings and the commandment to love another as ourselves. These two elements represent the two major categories of mitzvos; between man and God, and between man and man.
The offerings are between man and God. Loving another individual is between man and man. Therefore, when we say that Sefer Vayikra contains the majority of Torah, it is true on both sides of the coin. The majority of mitzvos between man and God is found within the numerous offerings, and the majority of mitzvos between man and man is wrapped up in the Torah's instruction to love a fellow person.
Therefore, since Sefer Vayikra contains the majority of both categories of Torah, and since the Torah is called "good", it is not surprising that specifically Sefer Vayikra and its "light" are called "good."
There is a second approach which explains why Sefer Vayikra is called "good." This is because teshuva (repentance) is called "good" (Yalkut Shimoni, Tehillim, #702, Psa. 25:8). Back in the day, repentance required offerings as well. Therefore, since Sefer Vayikra deals with offerings which are meant to motivate the person to teshuva, and since teshuva is called "good", it is not surprising that Sefer Vayikra is called "good."
There is also a third approach as to why Sefer Vayikra is called "good." Nowadays we cannot bring the offerings due to the absence of the Temple. The only way through which we can be credited with bringing the offerings is by studying the sections in the Torah that deal with them (Menachos, chap. 13, "Harei Alai Isaron", pg. 110a; Reish Lakish and Rebbi Yitzchak; Lv. 6:2 & 18).
When we keep the offerings in our minds by studying them and when we yearn to bring them, we are credited with having offered them. This is called having a "machshava tova" (a good thought) which Hashem is "mitztaref l'ma'aseh" (regards as a good deed; Kiddushin, chap. 1, "Ha-isha Niknis", pg. 40a).
The majority of Sefer Vayikra talks about the offerings that we cannot bring at the present time. The only way to "fulfil" the mitzvos in Sefer Vayikra would be through a "machshava tova" (a good thought). Since fulfilling the mitzvos in Sefer Vayikra is done primarily through "good" thoughts, it is not surprising that Sefer Vayikra is called "good."
Perhaps we could try to connect to the offerings a little bit more. We could choose to recite at least one Torah paragraph a day which speaks about the offerings. It would be a good idea to start with the Tamid (Daily Offering). Say it. Understand it. Desire to bring it. Then ask Hashem to accept the utterance of our lips as a compensation of the bulls (Hoshea, 14:3. See the prayer "Ribon Haolamim").
This practice incorporates all three points mentioned above. This paragraph about the offerings is "Torah" which is supposed to motivate us to "teshuva", that we are credited with offering just on account of the "good thought."
This practice could also set the pace for the entire day because it trains us to constantly think of doing good. Good thoughts typically lead to good actions. However, if we are prevented from carrying out the action, we are still credited with having done it just because we intended to do it.
So, may we all be blessed to heed our "Vayikra" (calling) in life by fulfilling every single mitzvah in the Five Books of the Torah, if not in action, then through thought, speech, and a desire to do them; thus filling the world with all the "good" holy lights, and subsequently deserve to bring actual offerings in the Third and final Beis Hamikdash.