"The Perfect Matrimony" and "The Mystery of the Golden Blossom" (the latter more specifically) describes in detail the essence of the spiritual matrimonial work. These are wonderful texts to begin with, especially the former if you want a more thorough explanation about all aspects of the teachings (since "The Perfect Matrimony" has everything in Gnosis, while "The Mystery of the Golden Blossom" is specific in the many nuances, particularities and practices of White Tantra).

As for sexual transmutation for single people, you can refer to the referenced lecture "The Transmutation of the Sexual Energy" at the bottom of this post, as well as the exercise of Christic Egyptian Pranayama found in "The Yellow Book."