At least three paperbacks were issued for The Invaders in the USA by Pyramid Books. Unusually it appears that more books were issued in the UK than the USA for this American series, while the titles issued in the UK appear not to have been published in America. Likewise the America titles appear not to have seen print in the UK.
This one is Army of the Undead by Rafe Bernard, published in Dec 1967.
Original Price $0.60