Anything that makes you
smarter while
you weren't paying attention. This can apply especially to children's books, like those written by
Holling C. Holling and, say,
Marguerite Henry.
Usborne books could be considered sneakily educational, but only if you're a book freak at age 10 and read anything with pages. I think a lot of Science Fiction is sneakily educational.
Things that are sneaky about teaching, are, in my humble opinion, way more useful than overtly "educational" things like textbooks and PBS shows. Although Nova can be pretty cool.