The Best of DC Blue Ribbon Digest is just that: reprinting the best storylines from Batman, Superman, World's Finest Comics, Superboy, Green Lantern, The Flash Barry Allen and featuring the Secret Origin of Super-Villains, the Justice Society of America, Legion of Super-Heroes, Teen Titans and other characters.
The digests, similar to Archie Comics digests, were published at a time before graphic novels and trade paperbacks, when publishers like DC Comics and Marvel reprinted material in comic book pages. DC's 100 Page Comics featured 10 - 12 pages of original stories and the rest were reprint material. Case in point The Best of DC #9, collecting the Detective Comics "Bat Murderer" storyline. Each segment is roughly 12-pages long. Combining that with a 1969 reprint from Batman, and a layout of the Wayne Foundation Building, what you have is a 100 page digest for under a dollar!
After 71 issues, the series ended in 1986.