This is a great place to find cheap used novels or non-fiction, theory and philosophy books, but the new selection is also quite good. Or, you can also order books (or even wine) overnight (this is where I ordered most of the Hegel books that I got in Heidelberg… in under 24 hours!). The selection is good and Stephan (the owner) is helpful and knowledgeable.
The location is rather compact, but the wall-to-wall stacks of books give it the right Buchhandlung atmosphere. It makes for a great after class Zen-book-perusing-fix stop on the way back from the library or the old city where I have occasionally made some great finds: alas, I never made it back to pick up that used Suhrkamp edition of Negative Dialektik! The philosophy, sociology, contemporary topics and politics sections are pretty good and the place generally has a left-leaning inclination (there are also events and presentations occasionally in the evenings), so check it out if you are tired of the Hauptstrasse!