("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, and on the first few pages, the dye from the page edges has bled in a little bit. But no writing or dog-ears on the pages, and the book is quite square for a book of its age and use. This may be a collectors' item thanks to what appears to be an off-plumb printing (see the pic of the back cover)?