At least four paperbacks were issued for The Invaders in the UK by Corgi/Souvenir press. 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 The Night of the Trilobites by Peter Leslie, published in 1968.
Original Price 3/6d