1977 book by Dr. Charles Silverstein that did for gay men what The Joy of Sex did for heterosexual couples. A second edition, The New Joy of Gay Sex, has been in print since the early 90's.
Topics include a broad range of gay male sexual practices, including male genital health and safe sex. I've only seen the "new" version, which has moderately cheesy illustrations. I'd like to check out the '70's edition to see whether it gives the super-cheesy drawings of The Joy of Sex a run for the money.
funny aside: amazon's displaying this book with a little crayon-style "to dad" icon, and a reminder that I can get it by father's day for only $3.99 more than standard shipping