The American Institute of Architects Bookstore

The AIA is the professional organization representing Architects. They have a very nicely designed storefront on Washington Avenue that functions both as their office and a bookstore /​gift shop.
They stock many books that would be of interest for architects or those interested in architecture. But they also carry a nice array of gifts and objects d’art related to architecture. As an example, they have some nice postcards, 3d building puzzles, models, paintings, etc. They tend to feature artwork done by local architects such as sculptures or photographs.
Anyway, it is a nice little place to take out-​of-​towners if you are downtown — especially if looking for some St. Louis keepsakes that you can’t find anywhere else in town.
As a bonus, it gives you an excuse to walk around in the lobby of the Lammert Building, a restored gem of a turn-​of-​the-​century office building downtown.

This is an odd duck in terms of gift stores. Apparently there are a lot of architectural firms in the vicinity of this place that might create enough of a demand for a store like this. Surprisingly enough, there’s a lot of cool things for sale, like an architectural history of St. Louis, as well as some diagrams and plans for new developments in the area. This feels like a functional gift shop, where information is sold, rather than trinkets.
However, you can buy trinkets, but of a more sturdy quality. They have some really cool St. Louis snow globes available, and some architectural models for the more adventurous hobbyist.
I would suggest this to Downtown visitors who are acclimating themselves to St. Louis. It’s actually quite informative to glance at the architectural history of the area before venturing off into other parts of the city.