The central government’s Council for Cultural Affairs has a small bookstore that sells material published by the council: books on Taiwanese traditional architecture, historical sites, birds, museums, flower arrangement, etc.
The store is at 30-1 Beiping East Road (the first large road north of Zhongxiao). It’s between Zhongshan and Hangzhou roads, much closer to the latter.
The books aren’t particularly cheap. I was, however, able to get a 20 percent discount.