Modern Sex Magick by Donald Michael Kraig gives a fairly decent overview of basic sexual magic practices, derived from a variety of sources, including those used by the OTO. There are physical exercises which are not very titillating and come off a somewhat clinical. But this is what is needed because sex magic isn't for fun, it is serious Work.
IMO, to really understand the OTO's methods a study of Vama Marga Tantra is really helpful. A book I that covers this from a good introductory philosophical angle is Tantra: Paths to Ecstasy by George Feurstein. The works of Arthur Avalon (Sir John Woodroffe) are a must if one wishes to pursue a wider understanding.
The works of Kenneth Grant are also useful for inspiration. Though his work was rooted in Crowley's system, he gives takes cross cultural synthesis to far out levels. He tended to link everyone to the 93 Current though, which can be a good baseline if you understand his type of reasoning. My understanding is that he also exposed the world to buddy Spare, Nema, and others.
I have yet to get into the works of Michael Bertrieaux (also introduced to me by Grant), but what I have skimmed so far seems pretty intense and motivational.
In conjunction with Yazata's recommendation of Randolph's classic work, the writings of Maria de Naglowska are also inspiring for their symbolist approach.
So far I have only read one work produced through the Temple of Ascending Flame, but I can say that, if one has familiarity with Crowley, Grant and Spare, these seem approachable and worthwhile.
