("a true crime shocker by") Carlton Smith and Tomas Guillen. At publication time, "America's #1 Unsolved Serial Murder Case," and Penguin offered a fifty-grand reward for information, as seen on the cover. (Also seen on the cover: the predictable and inaccurate promise of "chilling" photographs.) This book was a foundation true-crime book for a lot of people.
That might explain why you don't see many copies in the wild, at least in this edition. This paperback first printing has a cracked spine, some edgewear and rubbing to the covers, foxing to the page edges, and some mysterious writing on the flyleaf. But the book is quite square for a book of its age and use.