Question

What synonyms could I use for "virtual book"

Answer 1

online book ? maybe

Answer 2

audio book ?

Answer 3

online textbook.

Answer 4

You could consider being more specific with the type of virtual book you're talking about, to avoid repeating the phrase "virtual book".
For instance, there are pdf files, Kindle books, online versions of books--maybe even audio books count as "virtual books", arguably. It's also possible to say "electronic book" or "e-book".
In general, I think being more specific with the type of virtual book you're talking about might be the way to go, depending on the context