Opam (the CPAN/PIP) counterpart will handle the library cases fine.

Yes but it is not really relevant for that book. OCaml has an incredible backward compatibility history, and the linked book is about learning the language itself, not its ecosystem.

Things are different for RWO since it relies also on a number of extra tools and libraries which, indeed, change over time (sometimes also substantially)

The OCaml Software Foundation recently gave the author a grant to update the code in the book to the latest release of OCaml, 4.14, and release the new version of it online in HTML and PDF versions.

You can read it for free online at https://johnwhitington.net/ocamlfromtheverybeginning/