Way back when, DC experimented with their format in several of their regular titles, replacing selected issues with '100 Page Super-Spectacular' versions. They carried a significant price bump - 50 cents - but the page count certainly made them worthwhile. These typically didn't have any original content outside of new covers and letter pages, but were loaded with choice material from the DC vaults.
I've long suspected that the primary raison d'Ítre for these sporadic issues came out the need to maintain shipping schedules for newsstand distribution in the face of missed deadlines. Having fully prepared brick-sized reprint issues tucked away in the filing cabinets would have given editors significant peace of mind, particularly with serialized stories being much less prevalent than they are today. If production problems or artistic delays meant that an issue couldn't be completed in time, it would have been a simple matter to slide one of these super-spectaculars into the production queue in its place.
From an assembly perspective, each issue carried a specific number: for example, DC-08, DC-15, DC-20, etc. As a result, each of these issues ends up being counted twice in most price guides, as an issue of the title it was published under, and as an issue of what has come to be called the 'DC 100 Page Super-Spectacular' series, using the DC-hyphen-number scheme.
After some months, these reprint fill-ins evidently proved popular enough in certain instances to warrant becoming the regular format for some books for a while. Detective Comics, Batman, Shazam (as per the strip at the top of the page), House of Mystery, Young Romance and others all enjoyed several consecutive months of the format, much to the joy of fans of the old stuff. The inclusion of some new material, roughly a twenty-page lead story, accompanied a slight price hike, up to 60 cents. It was still a bargain, of course; a couple of those Detective issues featured a lead Batman story in addition to the classic Goodwin/Simonson Manhunter backup, with the balance of the book filled with reprints.
All told, DC published roughly 110 of these bricks, totaling 11,000 pages.
The next stage in the thick-reprint-book life-cycle came with the digests. DC tried a few variations at first, an 80-page all-Flash digest released under the DC Special Series line, a trio of western-themed Jonah Hex digests, etc. These led to two ongoing digest lines, 'Best of DC' and 'DC Special Blue Ribbon Digest'. These were all-reprint books for the most part, with only the occasional bit of new material thrown in. A Teen Titans digest featured a New Teen Titans story with art by Carmine Infantino; a Legion of Super-Heroes digest had a new story - a sequel of sorts - built around a reprint of a much older Shooter/Swan story); a Green Arrow digest had him and Black Canary browsing through old scrapbooks, recounting past adventures; and so on. The general practice, though, was for the digest to be exclusively digests, built around different themes, usually individual heroes or genres.
Adventure Comics also enjoyed a revival in the digest format for a while, lasting around eighteen issues. Each issue reprinted, in sequence, two stories from the Legion of Super-Heroes, starting with their first appearance. The balance of each book presented reprints of other features, along with some new material. Don Newton contributed a few Marvel Family stories, and Bob Rozakis teamed with George Tuska and Alex Toth to present a serialized Challengers of the Unknown story.
It's kind of ironic - these digests weren't all that popular when they were first published, but they go for decent coin now - if you can find them, that is. It's interesting to note that it's the genre-based digests that are the most sought after: romance, western, and horror. Lower print runs presumably feed this to some extent, but I wonder whether this might reflect an untapped market demand. DC cancelled their digest titles in the late eighties (de facto leaving the supermarket checkout counter distribution channel), even as the comic book store channel was booming nicely. With the comic book stores in their current state, perhaps it's time for DC to get back into the digest market, to get a mix of older and more recent material exposed to different audiences.
During the mid-eighties, DC published a wide range of reprint sets on better paper, including, among others, collections of the ten-issue Swamp Thing run by Len Wein and Berni Wrightson, the Batman stories by Steve Englehart and Marshall Rogers that ran in Detective Comics, the complete Deadman series by Arnold Drake, Carmine Infantino, Neal Adams, and others, originally published in Strange Adventures, and Jack Kirby's run on New Gods. A few one-shots were scattered in among these, including one that collected the various Man-Bat stories by Frank Robbins and Neal Adams.
[Quick Man-Bat run-down for you, since Fred's used him in one of the strips: violating about nine thousand different FDA regulations, scientist-type Kirk Langstrom doses himself with a serum derived from the glandular fluids of bats. Instead of just giving him expanded sensory abilities analogous to those of various bat species, the serum mutates him into a hideous half-man, half-bat hybrid. This is why mandated FDA trials are a *good* thing, kids.]
More recently, DC has been publishing 'mock' editions of the super-spectaculars, including a full reprint of one of the original super-spectaculars that was an all-romance issue. Unfortunately, the retro fervor didn't extend to the price tags. Sixty cents just doesn't go as far as it used to.