The big things that can come back and bite you on defense gun mods are deactivation of safety devices, and lighter-than-factory-recommended trigger pulls. I doubt anyone on GlockTalk would be stupid enough to deactivate any of the safety mechanisms on their Glock pistols, which leaves us with the trigger issue. Space is limited for responses in GATE, but fortunately, there's an excellent debate on this matter that covers all the bases in the Carry Issues section, here:
On the books, if you're looking for rules of engagement for armed citizens, I'd suggest "In the Gravest Extreme" (Police Bookshelf). On the "how to shoot" side, I'd offer "StressFire" (Police Bookshelf) and "Gun Digest Book of Combat Handgunnery Volume 6" (F-W Publications). For general personal protection/crime prevention, I'd suggest "The Truth About Self-Protection" (Police Bookshelf). For CCW issues, I'd suggest "Gun Digest Book of Concealed Carry" (F-W Publications). Books can be ordered from Police Bookshelf at 800-624-9049, and from F-W Publications at 800-258-0923. All were also available at amazon.com last I knew.