la doc, et
Read this paper by Didier Rémy to understand the theory behind the language http://caml.inria.fr/pub/docs/u3-ocaml/ Using, Understanding, and Unraveling
The OCaml Language
From Practice to Theory and vice versa

An online book on OCaml, a bit old, still a good resource http://caml.inria.fr/pub/docs/oreilly-book/ Développement d'applications avec Objective Caml

http://ocaml.org/learn/books.html

Real World OCaml https://realworldocaml.org/ (full book as html)