I know that "en" can be the "des céréales" in "Je mange des céréales" "J'en mange" and I know it can stand for "one book" in "Je vais acheter un livre" "Je vais en acheter un". But I find it kind of weird in:
Personne n'a écrit un livre sur comment en lire.
(Nobody has written a book on how to read them [books])
It just doesn't look like it fits, because it's in a noun phrase. It's referring to the "livre", which I find very strange, since it feels like it's not referring to books, and instead something else. Does this mean I could say:
J'ai fait un film sur comment en faire
(I made a film on how to write films) (Sorry for similar example)