Under v1.8, a written_book object requires double quotes around the text in the pages items, thus: written_book->tag->pages->"Page text"

Under v1.7, this was not present nor required.

As a result, when a 1.7 save is opened in 1.8, although the item ID is correctly migrated to the new String format, the page text becomes invalid (as it is not surrounded by double quotes) and so does not display.

Books created in 1.8 have the double quotes, and can be viewed correctly.
Suggest that either (a) requrement for double quotes be removed, or (b) migration function that changes the ID to a string also add the quotes.

Attached image shows what you see if you attempt to read a 1.7 book under 1.8