A book of instructions for handling blood, skin disease, mildew and debt. It is the least read book in the Bible and the one that most rewards being read as what it is: a working manual, not a narrative.
What the book is
Leviticus is a collection of ritual and legal instruction set at Sinai, delivered almost entirely as speech from God to Moses. There is very little narrative in it. Two brief episodes interrupt the instruction — the deaths of Nadab and Abihu, and the case of the blasphemer — and both function as rulings rather than as story.
Read as a story it is close to unreadable, and readers who give up on it usually give up here. Read as what it presents itself as — a manual for people whose job was to run a sanctuary — it becomes coherent very quickly. The questions it answers are practical: what to do with the blood, who may enter which space, how to tell a contagious skin condition from a harmless one, what happens to land when a family loses it.
How it is put together
The first sixteen chapters deal with the sacrificial system, the ordination of priests, purity rules for food, childbirth and disease, and the annual day of atonement. Chapters 17 to 26 form a distinct block usually called the Holiness Code, marked by a different vocabulary, a repeated refrain about being holy, and an address to the whole people rather than to the priests alone. Chapter 27 is an appendix on vows and valuations.
The seam between the two blocks is one of the clearest compositional joints in the Pentateuch, and it is visible in translation. The Holiness Code speaks in a distinct register and returns again and again to a formula — I am the LORD your God — that the earlier chapters barely use.
The dating question
Leviticus is the core of what critical scholarship calls the Priestly material, and its dating is argued almost entirely from language and institutional assumptions rather than from datable events, because the book contains almost no events to date.
The long-standing majority position places the final form in the exilic or post-exilic period, from the sixth century BCE onward, on the grounds that the centralised sanctuary it assumes fits the second temple better than the first. A serious minority, working mainly from Hebrew linguistics, argues that the Priestly Hebrew is demonstrably earlier than the Hebrew of Ezekiel and Chronicles and therefore belongs to the monarchic period, with the Holiness Code later than the rest.
Neither case rests on external attestation, because there is none for this book. Both rest on internal evidence: vocabulary, syntax, and what institution the text takes for granted. That is a weaker kind of argument than a dated tablet, and this site says so rather than presenting a date as settled.
Where the external record touches it
Directly, almost nowhere. No inscription mentions Leviticus, no excavation confirms a ritual described in it, and the sanctuary it legislates for has left no archaeological trace that can be matched to the text.
Indirectly, the comparative material is rich. Hittite and Mesopotamian ritual texts, the sacrificial tariffs from Ugarit and the Punic sacrificial inscriptions from Marseilles and Carthage describe offerings using terminology close enough to the Hebrew that the categories can be compared. What that comparison establishes is that Leviticus belongs to a recognisable regional practice, not that any particular chapter is old or new.
This is the ordinary situation for most of the Bible, and it is worth stating plainly here rather than only in the cases where the record happens to be loud. A book can be historically important and have no external attestation at all.
A note on how it reads
The purity material is often described as arbitrary. It is not arbitrary, but its logic is not moral in the modern sense either. The categories track order and disorder — what belongs where, what crosses a boundary it should not cross — and blood, disease and mildew all sit at boundaries.
Chapter 25, on the sabbatical year and the jubilee, is the part of Leviticus with the longest afterlife outside religion, having been read into debt relief arguments repeatedly over the last four centuries. Whether the jubilee was ever practised is unknown; no record of its operation survives.
Related: the overviews of Exodus and Numbers, which carry the narrative Leviticus interrupts, and the method note on what external attestation means.