The Big Book of Holding is just that, a giant book that holds, and catalogs, everything you put in it.
"Where are the antidotes?"
"Flip to 'A'!"
I did something similar but it trapped memories inside the book, and people that wanted to read it got trapped inside until they relived the memories of each page. The wizard wants to enter the book to find a memory to store a back up spell book in, because who would think to look for his spell book inside another book?
Inadvertently they could use this memory book as a "Book of Holding" but they have to enter and exit the book to get it and takes them far too long.
"Some that die deserve life and some that live deserve death. Can you give it to them players? Then be not to eager to mete out death in judgement! Even the very wise cannot see all ends..."