Kama Sutra
The Kama Sutra is one of the oldest written books whose content includes sexual practices. Contrary to what people think, it's not exclusively a sex book. It contains a wealth of material about the society in which it was written (India) and was supposed to be read by both men and women, especially the former, before marriage.
In ancient India, Kama refers to everything encompassing love, desire, pleasure, and sexuality. The Kama Sutra is a collection of Love Aphorisms summarized by Vatsyayana. In the Kama Sutra, besides the positions we all know, you'll find the basic arts that every man and woman should master, descriptions of the types of men and women, courtship strategies, or classifications of types of kisses.
It's a recommended read, although preferably in an adapted version, as the direct translation of the original is a bit slow to read. You're sure to extract some useful ideas from it.
For all those more interested in the topic of the Kama Sutra than in the book itself, here's a link to a popular PowerPoint file (virus-free, rest assured), which you might have received in your email at some point, with different positions along with their pros, cons, and most notable aspects.