I read an article a few years before Salinger's death (can't find the article right now) that foretold the publication of these books.

Salinger had kept his writing habit his entire life. He'd simply stopped publishing.

There was a room in his house with several manuscripts in it. Some labeled "publish as is" and some labeled "needs editing."

They were all to be published after his death. This way he could share his works with the world without being around to deal with any of the criticism.