Edge of Time by David Grinnell, a pseudonym of Donald A. Wollheim, SF editor and founder of DAW Books.

1967 Ace paperback F-162 with gorgeous Kelly Freas cover.

The book is in terrific condition, with an unbroken spine. A previous owner stamped his name on the page edges--something I've never seen before--but that and a tiny dig on the bottom left corner of the front cover are the only real condition issues.