book module - can't delete text permanently

I'm not sure what you mean by a "saved book collection".

An explanation-

If you select photos (from a folder or Standard Collection) and goto the Book Module to start a new book- there is a Tab;

Click this tab and it creates a "Saved Book Collection" in the Collections panel: (and the Tab disappears)

Always re-open this specific book design from this Collection, and never by clicking the top Menu for [Book]

You can re-open your Book by Clicking once on the white arrow or Double-clicking the Book Collection name.

Note the collection icon that looks like a small book.

And ALL future edits to this book are automatically saved in this "Saved Book Collection".

I would think that from the [Edit] menu there are two choices-

"Select All Text Cells" does delete all text in ALL text cells when I press [Delete]

"Select All Photo Cells"  (Do not press [Delete] ) but instead uncheck the Photo Text box in the 'Text' panel- This turns 'off' the Photo Text thus "hiding" Photo Text on ALL photos. 

I have no answer for the text re-appearing, but are you re-opening your book from a 'Saved Book Collection"? This is most important, as any book edits are 'saved' in the Book Collection.