These classics about controlled intercourse are available here. You can either read them completely online by following the links, or download and print them out:

The Karezza Method (entire text) by J. William Lloyd (1931)*

This book, written by a man, is a beautiful, inspiring manual about the practice of Karezza. It's full of practical information and highly recommended. (PDF version)

Karezza: Ethics of Marriage (entire text) by Alice Bunker Stockham, MD (1903)*

Stockham was a Quaker and one of the first women graduated from medical school in the USA. Her book is less user-friendly than Lloyd's, but also very inspiring. She spends a lot of effort on the larger spiritual implications of the practice. The letters from those practicing Karezza, found near the back, are particularly valuable. (PDF version)

Male Continence (entire text) by John Humphrey Noyes (1872)*

This is a pamphlet that Noyes prepared after 25 years of experience among an entire community with the practice he called "male continence." He stumbled upon the practice himself, after vowing not to put his wife through another agonizing pregnancy. He includes practical tips. (PDF version)

Sex Perfection and Marital Happiness (excerpts) by Rudolf von Urban, MD (1949)

Von Urban was a Freudian psychiatrist who counseled couples who were planning to divorce. Often he was able to help them save their marriages with his "six rules for human sex relations." The chapters included here address the separation problem between the genders, the bio-energetic aspects of sex, and masturbation. He also furnishes many case studies of couples in trouble who experimented with his advice. Very practical.

*As best we can tell the copyrights of these works have expired, so they are free for everyone.

Also see these other source materials from various traditions.