Yeah, definitely don't play with the BookInfo cache, it's not doing what you think it does (like, at all)

.
It's hidden seventy nesting layers deep for a reason ^^.
(The only non-developer valid use-case for messing with that is if you've massively changed your Library's content, and want to purge old thumbnails from no-longer-available books... to end up saving, like, 5MB.
TL;DR: Don't.).