MORE LETTERS--Assembly Speaker Willie Brown, in a personalized letter to his friends, has invited them to attend a Dec. 1 Century Plaza event "commemorating the 5th anniversary of my election as speaker." There's a gourmet dinner, special guest, music for dancing and "my sincere desire to share this evening with you." The tab? $5,500 a table. P.S. "Our Assembly leadership and I have designated this dinner as our singular most important event of this year." Of course, but what about '86?