"First, I know nobody who uses the word "book" to refer only to an electronic document, [...]"
It can be argued that "book" primarily refers to the information - not the medium. A scroll of parchment or vellum contains information that can be a "book". The Shorter Oxford Dictionary has "a writing" as its first entry for the meaning - and then goes on to fill a column with variant meanings.
As to pronunciation - English is notorious for breaking any apparent rules of phonetics for particular groups of contiguous letters. Over time some letters have become silent in certain combinations - but were previously pronounced . There is now a silent "k" in "knife" - but we would still pronounce it in the proper name "Knut".
The word eBook probably started life as e-book. The word "email" is now accepted as replacing "e-mail" - even though "emailed" has a prior meaning for enamelled or embossed decoration.
Therefore p-book or pBook can become valid differentiators for the medium of a paged book with paper leaves.