Magazines - Flying, AOPA, EAA pubs. As new tech, regs, and best practices emerge (which are usually decades old stuff, just renamed) the aviation press is quick to write about it. I've used King's instrument refresher, but that's more memory lane than building new knowledge.
Honestly, the primary body of book knowledge for GA is pretty finite, unless you like to delve deep into weather, aerodynamics, electrons, adminsitrivia, etc.