I know physical books are becoming an old fashioned thing but I always loved flipping through reference books. Nature, science... my grandmother had a great one about "the last misteries of the world" which featured, among others, the easter island statues and the nazca lines.
I have been thinking lately it would be great to find a reference book for my kids about world cultures. Something to give them that general knowledge. Like the tribe that puts several rings on their necks, know what I mean? Different cultures and maybe even religions.
If you know something like that let me know 
If you know other kind of amazing reference books I would love to know as well