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This book is free online: https://dev.realworldocaml.org/

I like Exercism for coding exercises that have runnable test cases: https://exercism.io/tracks/ocaml

See https://www.cs.cornell.edu/courses/cs3110/2020sp/textbook/

Based on the introduction, that looks like a great book which I definitely wouldn't have found on my own, so thanks for the suggestion! I'll have to give it a try when I have some time.

I have some more older links, ocaml changed a bit over time, but I think they are still relevant:

https://caml.inria.fr/pub/docs/oreilly-book/html/index.html

https://caml.inria.fr/pub/docs/u3-ocaml/

https://ocaml.github.io/ocamlunix/

I struggled quite a bit to pick up Haskell. OCaml was comparatively a breeze to learn, and a lot of fun too.

If you're competent in Haskell, and you've ever done any lisp, you should feel right at home with ocaml.

The other posters mentioned good books already. Another resource is Emacs + merlin, it really is magic when working with ocaml.