These classics about controlled intercourse are available here. You can read them online by following the links, or, in some cases, 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 or PDF version in a colorful, modern format.)

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 (scan of actual book) or PDF version, reduced to 26 pages)

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.

Love, Sex & Nutrition (excerpts) by Bernard Jensen, MD

Short excerpt from Dr. Jensen's book, in which he describes the benefits of Karezza.

Inner Studies: A Course of Twelve Lessons (excerpts) by Dr. Otoman Zar-Adusht-Hanish (1902)*

Excerpts from Hanish's book, in which he describes the benefits of sexual magnetism, and the need for self-control, except when conception is desired.

*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.