definitely Harrington on Hold'em. it seems like everyone's read it now and u don't wanna miss out on it.
There's weekly Poker tournaments in our uni where around 80 people turn up each week.
seems like half of them have read Harrington on Hold'em already.
u might think that since everyone read it it's pointless reading, wrong,
it's definitely helped me in my tourney play
(i've proudly managed to come 9th 3 times in the past 4 weeks.....
good or bad? i'll translate that again, i came on the bubble 3 times in the past 4 weeks

)
i guess it sucks being in the bubble everytime but i definitely played my best poker recently after reading HOH.
also, u sometimes hear people discussing strategies according to their M's....
if u do'nt know what M is, read HOH so u don't miss out on the conversation
is Theory of Poker any good :S?
I only play no-limit, and it seems Theory of Poker is just for all types of poker...