Leviticus is the third, and thus central, book of the Five Books of Moses. As such, its content forms the core of the Torah; in this sense, the Books of Genesis and Exodus can together be considered its prelude and the Books of Numbers and Deuteronomy its postlude.
The Book of Genesis describes why there had to be a Jewish people living in the Land of Israel. There was an original vision for creation, an opportunity that was missed; this set into motion a downward spiral of history that made it necessary for G-d to isolate a faithful core of humanity--Abraham's family--to preserve, bear, and eventually re-announce His message to the world. The Book of Exodus describes how this family was made into "a kingdom of nobles and a holy nation," and how the mechanisms whereby this nation could indeed bring the Divine Presence down to earth (i.e., the Torah, repentance, and the Tabernacle) were set up. The Book of Leviticus records the details of exactly how this end is to be achieved.
This notion is eloquently expressed by the very first word in the book, from which the whole book takes its Hebrew name: Vayikra, meaning "and He called." The prefixed "and" immediately connects the beginning of Leviticus with the end of Exodus: "Moses could not enter the Tent of Meeting since the cloud had rested on it and G-d's glory filled the Tabernacle." Since Moses could not enter himself, G-d called out to him, thereby enabling him to enter and bear the experience of His Glory in order to hear His message. This indicates that the events recorded in the Book of Exodus were intended to set the stage for G-d to call Moses and convey to him the contents of the Book of Leviticus. Furthermore, the usual way the Torah opens its descriptions of G-d talking to Moses is with the ubiquitous phrase, "G-d spoke to Moses, saying." In the opening of the Book of Leviticus, however, before the variant of this phrase--"G-d spoke to him from the Tent of Meeting, saying"--the Torah informs us that whenever "G-d spoke to Moses," He first "called out to Moses," implying that His communications with Moses were not merely for the purpose of laying down His law for humanity, but in order to call out to us, imploring us to respond, asking us to treat the laws of the Torah not merely as dry obligations but as our common meeting-ground with Him. To emphasize this point, this opening phrase is not worded "G-d called out" but "He called out," referring to G-d's very essence, not to any aspect of Him that can be defined by any of His Names. It is G-d's essence that calls out to us in the Book of Leviticus.
Thus, although there is very little "action" in the Book of Leviticus, it is here that the real "action" takes place: the inner life of the individual soul and the soul of the community in their communion with G-d. It is significant that Leviticus is not only the middle book of the Torah but the third book, for the number three expresses the essence of the Torah. The Torah is composed of three parts--the Five Books of Moses, the Prophets, and the Writings; it was given in the third month--Sivan; it was given to a nation of three classes--Priests, Levites, and Israelites; it was given after three days of preparation; and it was taught to the people by three siblings--Moses, Aaron, and Miriam.
The number three signifies the synergy that results from the paradoxical but harmonious combination of the two elements of a duality, and this is the very essence of the Torah: it takes two opposing entities, the physical and the spiritual, and creates from them a third--the peaceful fusion of the mundane and the holy.
From Kehot's Chumash