This is a bit of a pickle. For Real World OCaml to be a great book for OCaml almost requires Core to become the stdlib. Core is a pretty nice body of work, but I'm not a user of it myself, and I've been unable to decide if I'd like it as the standard library. There is a very strong flavor to the style. I suppose the official standard library has a bit of an "exceptional" flavor, but aside from that I feel it doesn't impose anything.
Jean, for you example, the List.iter line using the standard library would be:
List.iter (fun x -> sum := !sum + x) list;
This kind of confusion is certainly not good for learning a language though! To resolve this, we need to clone a single person three times and send each into an alternate future: one continuing with standard library as-is, one with Core, and one with Batteries. Then bring them back so they can argue amongst each other about what was better. :)