This book is a gem.
OCaml is a great language – I regret not learning it when I had more free time on my hands. It feels very reminiscent of Haskell, with two major differences – it's strict and it allows side effects. Those make reasoning about behaviour easier and removes the need for all that monad transformer nonsense. It's also very feature rich and it even has objects (although they are not as prominent as you would except, given that the O in OCaml stands for "Objective"). It seem to be the functional language to choose if you're doing something practical. For example, Facebook is using it for Hack, Flow and ReasonML.
There is a trading firm, Jane Street Capital, that uses OCaml for all their stuff. They've completely rewritten the standard library and have a set of replacement libraries (Base, Core_kernel and Core). The book introduces you to OCaml through those libraries, which is not surprising, as it shares an author with them (Yaron Minsky). I don't know whether it's industry standard, but it's pretty good and worth learning.
The book starts with an introduction to the language, slowly accellerates with some examples of dealing with hashes, command line tools and JSON, and goes completely bonkers into parsing, lexing, asynchronous programming, FFI, the memory layout and the garbage collector, and finally the compiler frontend and backend. While the first chapters are quite casual, the final ones can be quite the challenge. Unless you enjoy PLT, in which case they are quite the treat.
On the down side, it did not feel like it covered all corners of the language. There are bits in the OCaml manual that were not explored in the book, and I was left with the impression that the language is deeper than Real World OCaml made me think. Still, maybe this is for the better – I got much more excited about seeing some applications than I would have if it covered every nook and cranny first.
If you're into functional programming and you don't know OCaml yet, you should definitely learn it. This is the book to start with.