Upvote:2
For a detailed commentary, please see the Introduction to Tacitus: Annals, on Lacus Curtius, from which you will find the following analysis:
The Histories were followed in 116 A.D.24 by the Annals (libri ab excessu divi Augusti); which, after a short introduction, open with the death of Augustus in 14 A.D., and closed in 68 A.D., not, however, at the dramatically appropriate date of Nero's suicide (June 8), but, in accordance with the annalistic scheme, at the year's end. The probable distribution of the books was hexadic, Tiberius claiming I‑VI, Caligula and Claudius VII‑XII, and Nero (with Galba) XIII‑XVIII.25 Of these there remain I‑IV complete, p235 the first chapters of V, VI without the beginning, and XI‑XVI.
Footnote 25 should also be consulted for the author's sources.