Besides the "Quints" (Arsenal, Augmentation, Runner's Companion, Street Magic, and Unwired), which are the Advanced rulebooks, just about everything else is good. Some key books may be:
Adventure
Dawn of the Artifacts (Dusk, Midnight, Darkest Hour, and Dawn) plus Artifacts Unbound.
Twilight adventures (A Fistful of Credsticks, Anarchy Subsidized, Colombian Subterfuge) plus The Twilight Horizon
Location
Seattle 2072
Sixth World Almanac
War!
Sourcebooks/Campaign Books
Shadowrun 2050 (if you want to get nostalgic about the early days of the game)
Conspiracy Theories
The Clutch of Dragons
There's a bunch of other books, but these are my (current) faves. Check out some lighter PDFs like Safehouses, MilSpecTech, and Way of the Adept too for gaming crunch.