Since I review everything that I read, I find that there are many older, out of print titles that I just never get to - one day I promise I will, but for now my own list of forgotten books is rather limited.
Find the Feathered Serpent by Evan Hunter is a book my grandmother swears is the best teen adventure read ever. I'm under strict orders to take special care when reading it because she wants it back! Published in 1952, Feathered Serpent is about a couple of teens who travel back in time to the ancient Aztec world.
Now, a lot of the other entries are mysteries (I haven't looked at all of them yet, but the majority seem to be) and mine is Sci-Fi, but Evan Hunter is the pseudonym for mystery author Ed McBain so I figure I am not too far from the mark.
Another one from me would be The Other by Nicholas Tryon. Published in 1971 it's the story of two brothers with supernatural powers - and a twist. I have yet to read the book. It was either one of my mother's few growing up or she bought it for me at a used bookstore, either way I am hoping that it hasn't been misplaced for all eternity! There is a film adaptation, it's a bit slow and not what I was expecting, but I'd really like to read the book now.
I guess I'll have to troll my grandmother's closets for more old ones - I know she's got a ton of stuff that belonged to my uncle when he was a kid. I inherited my mom's Peter Benchley novels (she swears she wasn't a reader, but I have the books to prove it!).