I don't sit at a desk. But on my table at home next to my bed are:

Mary Ann Caws: Manifesto, a Century of ism's -- which is very interesting if you like the history of ideas. It basically collects all the written manifestos of the various artistic movements of the 20th Century.

and

Mark Prendergast: The Ambient Century -- which is about the evolution of ambient music from the early 12-tone classical composers in the 1900's to today. He's somewhat vague about what ambient music actually IS, but he has lots of information about obscure 60's psych and folk bands, along with lots of other good stuff.