Even if Aunt Martha does not have hollowed out book on her bookshelf, any book in her collection could have an obituary, photograph, letter, or other paper-based family history item tucked into it. Family Bibles are the first place to look–and to page through page by page. Clippings, funeral notices, and the like can also be used as bookmarks.
Make certain you have flipped through all the pages of those books if you have the chance to go through them. You are looking for items of family history value–not just items with monetary value.
When the book in the illustration was pulled off the shelf, it’s purpose was clear. Not all hollowed out books are as easy to spot.