2. Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships by Christopher Ryan and Cacilda Jetha
4. Masters and Johnson on Sex and Human Loving by William H. Masters, Virginia E. Johnson, and Robert C. Kolodny
6. Sexual Behavior In The Human Male by Alfred C. Kinsey
WHY: Without Kinsey, there would be no Masters and Johnson. Kinsey's work, published in 1948, was highly controversial as it addressed masturbation, adultery, premarital sex, homosexuality, and bisexuality. Kinsey himself was bisexual and used himself as a subject to extract data firsthand, and while much more research has been conducted since, there is an astounding amount of information that still rings true today. He also later wrote a version for women, which was also groundbreaking.