The first edition of Real World OCaml is freely available online. It introduces you to OCaml, an industrial-strength programming language designed for expressiveness, safety, and speed.
Free OCaml Books
“Think OCaml: How to Think Like a Computer Scientist” is an introductory programming textbook based on the OCaml language. It is intended for newcomers to programming and also those who know some programming but want to learn programming in the function-oriented paradigm, or those who simply want to learn OCaml.