There are several outlets of Eslite in Taipei, but its flagship is definitely the one located at the Renai Traffic Circle. A bookstore opened 24 hours (actually the first in the world) where you are welcome to browse, but also to read comfortably as long as you want. They offer one of the best catalogues, not only in Chinese and Japanese, but also in English.
As a plus, in the B2 of the same building you'll find the best record shop in the city. Well... there are no many good CD shops left.