More old stuff dating back to the golden era, this is the second part of an Ace 'Double', a pocket containing two novels in one book.
The aliens, so different from humanity at the beginning of the book, blend in a little too easy. The whatever-the-name-enemy from another dimension is not convincing. And the story didn't age very well.
Your mileage may vary. But, unless you're so desperate to find out what SF was like back in... euh... 1961, then I would skip it.