Introduction to OCaml. Jason Hickey.
Allen Downey and Nicholas Monje.
OCaml Batteries included (or simply "Batteries") is a community-driven effort to standardize on an consistent, documented, and comprehensive development platform for the OCaml programming language.
Core, an industrial strength alternative to OCaml's standard library.
Musings from Jane Street's OCaml developers and Systems Group.
Xavier Leroy and Didier Rémy.