Space: 1999 spawned a very extensive series of spin-off novels including quite a few original novels, which included E.C. Tubb’s Alien Seed which appeared in 1976.
Alien Seed is one of the more successful TV tie-in novels that I’ve read so far. It’s slightly more serious in tone than the television series but it still manages to feel like a genuine Space: 1999 story. It’s quite ambitious, it's reasonably intelligent and on the whole I think it can be said that it's a succees. It’s definitely worth checking out. Here’s the link to my review at Cult TV Lounge.