I think this is a great quote and it was clever for the author to open up the book with this sentence. Holden doesn't want to tell his whole life story from beginning to end, like in the book David Copperfield. Instead, he wants to talk about "all the madman stuff," that happened to him around last christmas before he got pretty rundown. That's when he had to come out to Pency to take it easy. This stood out because it shows what kind of personality Holden has and it shows that he doesn't want to be full of himself. He just wants to tell the important stuff.
Why do you think Holden had to take it easy?