A: Their favorite book.
Again, I get very wrapped up in criteria when it comes to discerning these things. Sure, I could give you a great long list of books I like, or love. But favorites? That's narrowing it down to a microscopic thing.
So, I've got three.
Jane Austen's "Pride and Prejudice". I have lost count of how many times I've read this book over the last twenty years. I used to enjoy the complicated romance of Elizabeth and Mr. Darcy, but as I grew older, I appreciated other things: the genuine description of Regency England, the rules and standards in place for women and men of the time, the sort-of-sad-sort-of-not story of Charlotte Lucas and Mr. Collins. To me now, this book is like a comfy pair of sweatpants - totally old and familiar, but still a great source of joy and comfort.
Alice Walker's "The Color Purple". Though it's been awhile since I've reread this book, it's quite a powerful story of love, loss, redemption, friendship, and life. Okay, yes, racism is a significant component, but it doesn't eclipse the other themes.
Neil Gaiman and Terry Pratchett's "Good Omens". Most of the books I read are either deep and written as such or fun and written as such. "Good Omens" is what happens when a book is fun and deeply written. The topic is religion, namely the coming of the end of the world...but you know, done in a fun and extremely clever way. Trust me, you'll want to be one of the Four Horsemen of the Apocalypse (albeit a marginal one) too.