I've been reading a lot this summer; a lot of it has not been particularly Less-Wrong-themed, though.

For people who have a math, cs, or electrical engineering background, "A Wavelet Tour of Signal Processing -- The Sparse Way" by Stephane Mallat is a very useful overview of recent research that you'd otherwise have to chase through papers, written with more verve than any textbook I've ever seen, and yes it can change the way you see the world.

Other summer highlights:

Cryptonomicon, Neal Stephenson (essentially, built to spec especially to entertain me. Kinda dumb in places, but mostly fun.)

Vietnam, Stanley Karnow (the war was very, very weird. Can be a "rationalist" book in the sense that it's a historical exercise in how smart people can reason poorly.)

Breakfast at Tiffany's, Truman Capote (if you've only seen the movie, this is infinitely more serious and poignant.)

The Possessed, Elif Batuman (a memoir of life as a Slavic Literature grad student. Lots of memoirs have funny anecdotes, and so does this one, but what's special is that Elif also gets across her intellectual excitement.)

Exiles, James Joyce (yep, he wrote a play. I have a thing for Joyce and I read this like five times in a row.)

The Fortress of Solitude, Jonathan Lethem (a novel about growing up in Brooklyn in the 70's. If you want to know what great contemporary writing looks like, here it is.)

Generation of Swine, Hunter S. Thompson (I just love having his voice in my ear.)

The New Testament (long delayed but I really needed to read it. I've just gotten through the Gospels so far.)