The Book of Leviticus is the third book in the Old Testament of the Bible. It is, essentially, the Mosaic Law, the laws laid down by Yahweh through Moses. These include what not to eat, how and when to sacrifice animals to the Lord, what is clean and unclean, and what kinds of sexual activities are worthy of the death penalty.
In the first place, there is no affirmative evidence that Moses is the author of those books; and that he is the author, is altogether an unfounded opinion, got abroad nobody knows how. The style and manner in which those books are written give no room to believe, or even to suppose, they were written by Moses; for it is altogether the style and manner of another person speaking of Moses. In Exodus, Leviticus and Numbers, (for every thing in Genesis is prior to the times of Moses and not the least allusion is made to him therein,) the whole, I say, of these books is in the third person; it is always, the Lord said unto Moses, or Moses said unto the Lord; or Moses said unto the people, or the people said unto Moses; and this is the style and manner that historians use in speaking of the person whose lives and actions they are writing. It may be said, that a man may speak of himself in the third person, and, therefore, it may be supposed that Moses did; but supposition proves nothing; and if the advocates for the belief that Moses wrote those books himself have nothing better to advance than supposition, they may as well be silent. 
See also Numbers