The book is called Ideal Marriage: Its Physiology and Technique, and a librarian named (delightfully) Billy Parrott wrote about its late return at the NYPL's blog; the copy of Ideal Marriage was due August 1959, and was mailed from Arizona last year with an note:
We found this book amongst my late brother-in-law's things. Funny thing is the book didn't support his efforts with his first (and only) marriage... it failed! No wonder he hid the book! So sorry!!
A shocked in-law
After the jump, the Ideal Marriage Google Books word cloud, which includes my least favorite word of all time.