Heidi's sex report, Kinsey's sex report.
In ancient India, there was also an important sexual classic, The Sutra of Love. The famous Arabic classic "The Sutra of the Plain Lady in Xiangyuan" is the oldest existing sex book in China.
Indoor Examination of China in Past Dynasties (all three volumes)
This book gives a comprehensive and systematic exposition of China's past dynasties' sexual culture, and it is an exquisite classic that vividly and intuitively depicts the art of China's house.