OCaml 4.06 has, by default, immutable strings. This necessitates some changes to "More OCaml" ("OCaml from the Very Beginning" is unaffected).
The paper and kindle books (on Amazon), as well as the PDF e-book here, have been updated, together with the online resources. The new code works on any version of OCaml starting from 4.02.0.
If you are an existing customer, and have not received an update email, please contact the author using the form on this website.