"The Function Of The ORGASM" by Wilhelm Riech.
I read the whole book, but not sure you need to ...well, maybe you need to read it all, to get and make the commitment.
Great mental health through proper breathing and pelvic thrusts leading to orgasm ...hard orgasms ...in intercourse.
Pity the current edition does not have the same drawings and figures by Riech.
PS - For those into porn, CIM, COF, COB, or jerking, you might not bother with this book. You could get a paper cut turning the pages !!!
Since my read and study of this book, long ago, I have always been committed to CIV. Although I must admit that damn, 'The Ashley Experience' has overpowered my true desire on ocassion, but just from time to time.