The first season followed swiftly on Quinn Martins' successful The Fugitive, and no secret is made that they wanted another show following along those lines. So Larry Cohen cooked up the notion that a lone man, a witness would pursue the threat of alien invasion single-handedly and single-mindedly. The first season was as much a study in paranoia as a sci-fi thriller. While the viewer is reasonably confident David Vincent (played masterfully by the exceedingly handsome Roy Thinnes - according to my wife) has truly seen aliens, few of his fellow characters believe him and he himself doubts from time to time. Slowly and relentlessly, he uncovers threat after threat following the vaguest of clues around the country, and often stopping the menace. But while it's a compelling concept, it's alas a limited one. And even by the end of Season One, you can tell the framework of the series is changing.
Those changes are all too evident in the second and final season as David finds allies in his struggle against the aliens, a group of "Believers" who funded by an important and wealthy industrialist are able to up the ante in the battle against the invaders. Kent Smith plays the industrialist Scoville who becomes a virtual co-star through the remainder of the series run. Eventually the authorities are drawn into the secret war and by the end of the season it's hardly a matter of David's sanity, but merely a question of how effectively the human race itself will face this looming and growing threat. The second season offers up much more detail about the aliens themselves as we learn there are factions within their ranks, not all of them thinking the invasion is a good thing for anyone. They have a decidedly caste society with some few designated as "Leaders", of which only a half dozen have come to Earth. We get to see inside the iconic spaceships more than once, and the invaders themselves die in droves as they meet their maker by immolating in a bright red after death.
The Invaders was adapted to not only Big Little Books, but also to comic book form. Below are the covers to the four issues of the comic published by Gold Key. Here are some links which will take you to a site where you can read the complete issues beautifully rendered by the painfully under-appreciated Dan Spiegle. Just click on the issue numbers - go to The Invaders #1, #2, #3, and #4.
Also there were some paperbacks written by the likes of Keith Laumer and others which filled in details of David Vincent's battle. If you would like to check these novels out, all three are available at the Internet Archive at this link.
And finally, there's this Whitman gem, a novel which serves as a companion of sorts to the BLB. It's a great stark image of Vincent as he runs in terror from the approaching saucer. The evocative logo for the show is much in evidence on this one. This too is available at the Internet Archive at the following link.