Beware that 6502.txt has
many critical typos (like the wrong opcode listed in places). This is why I don't like to recommend it to people as a reference doc, even though it does a good job of explaining things.
I'd recommend you use obelisk as a reference to double check against 6502.txt:
http://www.obelisk.demon.co.uk/6502/reference.html
I've been using it for a primary reference for years now and I can assure it is error-free.
Though if you want details about what the CPU is doing on each cycle -- it doesn't get better than this doc (scroll down to near the end):
http://nesdev.com/6502_cpu.txt
That's only really useful for emulator devs, though.