Well, the ones I have re-read the most are my Harrington on Hold'em books. I would estimate I've read the tourny books approximately 5 times through, less the 3rd which I've only done 3 times.
The cash game books I've gone through in their entirety about 3 times right now. But my friend is borrowing them and when he finishes, it's going to be discussion time, so there will be intense review and debate on much of the material, which I quite look forward to.
I've also re-read ace on the river a few times. I like that book quite a bit. It's a change of scene so to speak amongst the plethora of hand and analysis poker books.
I also think that Sklansky and Miller's "No Limit Hold'em Theory and Practice" warrants multiple readings simply for my dumb brain to absorb a little more of the content each time. (I love math, love it! But boy do they go statistical at times)
If you're wondering which is my preferred read? It's Harrington on Hold'em Cash Vol 1 & 2. I'm a cash game player, and I just really like everything about Harrington's books. They're set up well, they read well, and getting a peek inside the thought process of one of the greatest strategical game players in the world is awesome to me.