If we're talking having the info in these books in our heads like a computer to access the information at will, then I'd have to say a stack of travel guidebooks. A stack of Let's Go books, instantly in my head, would be incredibly helpful when choosing our next travel destination as well as not to have to dig out the book to look at maps, find places to stay & eat, details on museums, etc.
Otherwise, I'd say a stack of mixed history books (world, art, etc). While I enjoy fictional books that take place in different times, to actually sit down and read a history book would be too dry for me.