The Book of the Secrets : by Osho, The original series of 80 discourses were simply called ”Vigyan Bhairav Tantra” published as ”Vigyan Bhairav Tantra Volumes 1 and 2. (free download in pdf format)
http://www.oshorajneesh.com/download/osho-books/Tantra/Vigyan_Bhairav_Tantra_Volume_1.pdf
http://www.oshorajneesh.com/download/osho-books/Tantra/Vigyan_Bhairav_Tantra_Volume_2.pdf
In this comprehensive and practical guide, the secrets of the ancient science of Tantra become available to a contemporary audience for the first time. Confined to small, hidden mystery schools for centuries, and often misunderstood and misinterpreted today. Tantra is not just a collection of techniques to enhance sexual experience. As Osho shows in these pages, it is a complete science of self-realization, based on the cumulative wisdom of centuries of exploration into the meaning of life and consciousness. Tantra-the very word means "technique"-is a set of powerful, transformative tools that can be used to bring new meaning and joy to every aspect of our daily lives.