Which bookstore chain is the setting of the episode 'The Bookstore'?

The bookstore chain featured in the Seinfeld episode “The Bookstore” is Brentano’s.

“The Bookstore” is episode 173 of Seinfeld and the 17th episode of season 9. It originally aired on April 9, 1998. The store provides the setting for the main Jerry-and-George storyline: Jerry catches Uncle Leo shoplifting, while George is forced to buy a book after reading it in the store’s bathroom.

Brentano’s was a real American bookstore chain founded in New York in 1853. Its name is sometimes confused with Borders, Waldenbooks, or B. Dalton, all recognizable bookstore brands, but none is the chain named in this episode. George’s attempt to retaliate against the store by stealing and returning another copy of the book becomes part of Jerry’s plan to divert attention from Uncle Leo.

The episode also includes Elaine’s “detox poncho” storyline and Kramer and Newman’s ill-fated rickshaw business, making the bookstore plot one of several loosely connected comic threads.