Havelock Ellis 
The Psychology of Sex 

This book was written almost 150 years ago, and yet it is just as relevant today. Havelock is transparent with his sources and data—which at times makes for lengthy passages—but greatly enhances the material. The reader can examine the data and determine if s/he arrives at the same conclusion.
I enjoyed Havelock/’s voice as a writer, and I found myself agreeing with his rhetoric more often than I disagreed. The depth of his research and his passion for the subject, notwithstanding society‘s preference to obscure rather than elucidate sexuality, is staggering.
On the whole, Havelock takes an extraordinarily complicated field and makes it both approachable and enjoyable. I highly recommend adding this to your collection!