There's not a lot of information on PLT Inc. these days, so: Racket was developed under direction of Matthias Felleisen, with his students Matthew Flatt, Shriram Krishnamurthi, and Robby Findler serving as the core development team. They also published the book "How to Design Programs" [0], and Krishnamurthi published "Programming Languages: Application and Interpretation" [1], which are both books that rely on Racket. (PLAI actually uses its own language which is developed in Racket, called plai-typed, but which relies on the DrRacket environment.)

[0] https://htdp.org [1] http://www.plai.org