I should begin this post by admitting immediately that I can only answer th efirst half of your question ("What is it?") and not Crowley's use (or abuse) of it.
Sex magic is used by several religions - in Tantra (what I am familiar with), we are taught that with the mindful unification of the male and female force (not just their genitals) a greater power for both to experience is released. Yes, I've practiced it - alone (yes you can practice by yourself) and with my spouse. Yes, it works if you treat it seriously and practice the meditations mindfully, you will notice your sex life (esp. your orgasms) changing.
There are many different kinds of meditations, as well as practices, in order to tap into this source inside us. Some folks have heard of ejaculation control and other methods, to build and control sexual energy within the self, but in my opinion (from the reading I've done) the emphasis on retention of fluid and delay of gratification is over-emphasized in current thought. The oldest (left-hand) Hindu traditions do not emphasize these practices - it's my belief they may have originated from early Chinese philosophy.
Older sex magic traditions are linked to the Isis/Osiris mythology of the ancient Egyptians as well - these I only know of in passing, but cannot speak on myself.
Books I would recommend:
On Tantric philosophy:
- Desire: the Tantric Path to Awakening by Daniel Odier. Instructs a new reader to the basic Tantric concepts, including that ITS NOT JUST SEX, and that the power of Desire is far more potent a spiritual source (as opposed to just "lust"). Emphasis on the singular practice of Tantra, as opposed to couples-only materials.
-Tantric Visions of the Divine Feminine: the 10 Mahavidyas by David Kinsley. Kinda dry reading, very analytical, but examines the faces of the female sources of power in both Hindu and Buddhist religion (Kali and Tara are discussed first, after that are dieties most Western readers don't know of).
On Tantric practices (or, what can me and my beloved do together?):
-Tantric Love by Ma Ananda Sarita.... You really have to get past the New Age nonsense here and the goofy pictures of our naked couple in this book, and see past the cover to the content. It's full of very good exercises to enhance a couple's ability to connect, and the exercises really will change your sex life, including your orgasms (if you can't wait, go straight to the "Khajuraho meditation" and start practicing with your beloved, but don't expect instant results, it takes a few weeks of regular couples-meditation practice so be good about keeping up with it often).
On all around Tantric info:
- Complete Idiot's Guide to Tantric Sex - my hubby's favorite, a typical "Idio'ts Guide" in that it gives lots of small information from all over, but nothing helpful (meditations, etc) to those who wish to devote themselves to the practice.
On Sacred Sexuality:
- Sacred Sexuality by AT Mann - Pretty book, examines the sacred practices of sex magic in many cultures around the world, with pictures othe culture's art, scultpures, etc. that celebrate the traditions.
- Sacred Sexuality in Ancient Egypt - Haven't read this one yet, I must admit. I bought it years ago and then never got around to reading it, but if you wanted a title relevant to Egypt....
As for Crowley's own involvement, I'm afraid I'm too ignorant to answer. By reputation, I've heard Crowley abused sex magic practices to coerce others into bed for his own personal gratification. But I have no idea if that's true, and I've not studied Crowley (yet) in my life.