The Invaders was a very popular Sci Fi series from the 60s. In the US three books were published by Pyramid Books and in the UK four books published by Corgi Books. The UK title "The Halo Highway" was a reprint of the US title "Army of the Undead". Also, the UK title "The Meteor Men" was a reprint of the US book No 1.
In adition to the paperbacks, two hardback books were published by Whitman Books in 1967, one in their Big Little Book range.
US - Pyramid Books