First, I'm amazed by how many people have suggested Brian Eno's ambient music, especially "Music for Airports." And unfortunately, Amazon does not have it for sale new and it isn't in the iTunes music store. (Note: I'm not trying to prompt people to mail me sound files. Please, don't.) [UPDATE: Now, for some reason, I'm finding it on Amazon: here.]
Second, though some have recommended classical music, one person warned against classical music, saying:
Do not be fooled by anyone claiming to have a list of classical pieces that foster easy reading. Classical music is so complex and interesting that it claims your full attention. You will be too busy wondering what's going to happen next musically to devote any of your brain to your reading.
I think this is absolutely right. I bought a CD of the supposedly "relaxing" classical music for my work computer to screen out noise, and it is distracting. For years, I relied on this CD of much older music and found it very helpful. The emailer that warned me off classical music recommended mediaeval chants.
Third, many people have recommended film various film soundtracks -- I'll name some later -- and I think one reason many of these are good is that they are similar to classical music, but much less likely to be distractingly interesting, surprising, and complex.