Not really a book, but a transcript of the commencement address he gave to Kenyon College a bunch of years back.
Very thoughtful and inspiring, and a call to get us to be more attentive and aware to others, and not to simply exist within ourselves, our default setting.
If you don't want to spend the money on the "book", you can find the audio of the 22-minute speech (in 2 parts) on YouTube. I listened to those as well.
I very much recommend listening to/reading what DFW had to say here. It's solid stuff.