These were bound-together remaindered copies of selected DC and Marvel comics without covers. Very hard to find in high grade but often show up on eBay in low and mid grade. Good reading value.
To my knowledge there are 24 different Double Double Comics across a number of titles - Action, Adventure, Batman, Detective, Jimmy Olsen, Justice League, Lois Lane, Strange Adventures, Superboy, Superman and World's Finest. But I'm still looking!
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.