Dr. Schwartz: Yes, there are several. The Joy of Sex shows all kinds of positions - the first edition has drawings while the second edition shows photos. Or Anne Hooper has a number of books with photos of a variety of sex positions. There is also a fun book called The Guide to Getting it On! that has detailed instructions and drawings of options. I think you will find many delightful alternatives to using your knees.
For a real treat, here's a suggestion to use on vacation: go to a resort or rent a house with a private pool and find ways to use weightlessness to your advantage!