The Secret Visitors

From Author Wars - https://authorwars.com/publications/the-secret-visitors-241169.html

Overview:

Title: The Secret Visitors
Author: James White
Year: 1961
Publisher: Digit Books
Price: 2/6
Pages: 160
Binding: Paperback
Type: Novel
Title Reference: The Secret Visitors
Notes:
"A Digit Book" "© A.A. Wyn Inc., 1957" "First published by Ace Books, New York" Undated edition. Tuck says 1961, but also claims it's #D479. The same cover was used here and has Emshwiller's initials.