I did read (and comment a bit) on the 'Real World Ocaml' beta book, but I've already known and used the language for quite some time before, so I can't really judge which book would be better for a beginner. Take this with a grain of salt:

"OCaml from the very beginning" seems to focus more on teaching the language itself. If you've never used OCaml before then this might be the place to start.

"Real World OCaml" in addition to teaching the basics of the language has some intermediate-level chapters (dealing with json, S-expressions, asynchronous events, parsing), and some advanced-level chapters (GC, compiler frontend/backend). I definitely recommend reading it at some point.