Shameless plug: there's also a newish O'Reilly book, Real World OCaml http://realworldocaml.org, which I think is a big help in learning the language.
Oh, and there's OCaml Labs, a new lab at Cambridge University that's dedicated to improving the language.
And of course the compiler is constantly making progress. The upcoming 4.02 release is a pretty fun one, which I documented here: https://blogs.janestreet.com/ocaml-4-02-everything-else/ And before that, changes like GADTs and first-class modules landed, which have been quite useful extensions.
Really, it's a very active and fun community these days. The language is getting better quite quickly, but mostly in conservative and tasteful ways. The people in charge of the core language have been doing a great job, and the community infrastructure (things like OPAM) have been making big strides as well.