These were bound-together remaindered copies (without covers) of selected DC and Marvel comics with covers taken from original DC comics or sometimes new covers adapted from interior scenes. Very hard to find in Near Mint but often show up on eBay UK 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!
Aug 2011: a VG copy fetched an amazing £34.99 on eBay UK.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.