"Beach Boys biographer Jon Stebbins tells the tale of the song in his Web site's FAQ. It seems that Preston and Wilson were at a party late one night fumbling with songs, and Wilson helped him complete it. If you check out Wilson's solo work, you can hear where Wilson's mind took over 'Beautiful.' Wilson never pressed the issue, and let Preston have the cut and the royalties."
Craig Hlavaty in a 2012 Houston Press article explores the "Secret History of 'You Are So Beautiful'."