Anyone else try to check the books on their shelves? (I really couldn't make out any.)
Alice Walker - the open book looks like either the Oxford or Cambridge dictionary (I don't have them near, so I can't check), but one or other has the funky bits cut so you can skip straight to the letter you need.
Henry Miller - hard to make out in the photo, but there is footage somewhere of the bookshelf which is clearer. There's mention of collections of short stories that I remember, but I wouldn't like to say for sure which ones he read.
Mark Twain - he wrote a bunch of book reviews, so if you look through those you'll probably discern which he kept and which he got rid of.
From reading about Agatha Christie, I'm willing to bet that a lot of the books on her shelves are reference books rather than novels. Too small an image to pick out any that stand out.