There's something inherently old-school about digest-sized game books. That's because the earliest games, like OD&D, T&T 1st ed., and CT were all first issued in digest-sized volumes. I wish more games were printed in digest size, not only because they'd be cheaper, but because that would just be cool. Here's a few digest sized games I have...
Published by Corgi Books and distributed in the box set of Tunnels & Trolls, this digest sized rulebook is a reprint of the venerable 5th edition rules with only minor alterations (but not the 5.5 rules). The paper isn't the best quality, and it's missing the illustrations by Danforth and Carver that make 5E so memorable, but this is still the rulebook I get out when I want to play a game.
It was cool how Guardians of Order printed most of their books in digest format. It wasn't until the end that they started releasing big, expensive hardbacks (which may have contributed to GoO's demise, but I could be wrong about that). Of course BESM kind of contradicts my assertion that digest size books are old school, since it was a decidedly new school, story telling game. But it was still cool.
Slightly smaller than digest sized, the Pocket Edition of Traveller (Mongoose) is a miniaturized version of the main rulebook. Unfortunately that means the text is very tiny, so reading these rules involves a lot of squinting. I really like the Mongoose iteration of the classic Traveller rules a s they address some of my nit-picks with the original and more importantly they combine all rules in one volume rather than splitting then up into three volumes.
One of the reasons the first rpgs were digest sized is because that was the format of the miniatures wargames that the hobby grew out of. Some wargames kept that format, like Shock Force 2nd Ed. This fast, fun skirmish game is set in a science fantasy setting that's somewhat reminiscent of Gamma World, but the force builder rules let you take this game into any setting...like Gogol Island, for ex.
Another great set of skirmish rules I have are Wessex Games' Firewall 2136AD. While it doesn't capture the feel of cyberpunk as well as Cyberpunk 2020, it's still a fun rules-lite game.
UPDATE: I can't believe I forgot BESM at first. I added it and a few minor clarifications and corrections.