Raise High the Roof Beam, Carpenters and Seymour: an Introduction is narrated by the fictional Buddy Glass, an English professor and professional writer. "Seymour" in particular is about the writing process, and specifically the difficulty of writing about a subject one loves and admires as dearly as Buddy does his brother Seymour. In its discussion of communication in general the book suggests that languages obscures truth, and that communication is necessarily flawed.
Salinger defines poetry differently in "Raise High the Roof Beam, Carpenters" than he does in "Seymour: an Introduction."
We can only fully understand Salinger's definition of poetry by reading these two short stories together. The ideas, which are introduced in "Room Beam," are completed in "Seymour."