Like many of Salinger's Glass family stories, both "Raise High the Roof Beam, Carpenters" and "Seymour: an Introduction" are filled with intense admiration. We have admiration on the part of the author for his fictional characters, and also on the part of the fictional narrator/writer (Buddy) both for his own story telling and his subject matter (his brother Seymour, in this case). "Seymour" deals most explicitly with the artistic consequences of such intense admiration and emotion. Buddy finds that he cannot write with accuracy about a man he holds in such high esteem.
Buddy is blinded by his own closeness to and admiration for Seymour; we can't expect a realistic depiction of Seymour through Buddy's eyes, and yet we are limited in our conception of Seymour to Buddy's recollections, opinions, and musings.
Because we get to read Seymour's own words (through his diary and notes) and hear about his own writing (through Buddy's summary of his poetry), we are able to get at the true nature of his character, despite our narrator's bias.