I don’t know if it counts as a getting started guide, but there is a very good (if somewhat opinionated) book aimed at beginners available for free online:
- Real World OCaml https://dev.realworldocaml.org
The following book is a bit outdated (it is pre-OPAM), but it is also very good:
- Developing applications with Objective Caml Developing applications with Objective Caml
The next book (in french) is even more outdated (it dates from when there was no “O” in OCaml), but if you are willing to put up with that, you will be rewarded in spades. In my opinion, one of the best books to learn Caml and programming anywhere:
- Le langage Caml https://caml.inria.fr/pub/distrib/books/llc.pdf
Finally, and on a more specific subject, if you are interested in classical Unix programming you have the excellent
- Unix system programming in OCaml Unix system programming in OCaml
Cheers,
Nicolas