Here are the covers to the two UFO novelizations published by Warner Books in the United States. I actually have both of these, picked up at a yard sale in the mid-80s, if I recall correctly. At the time, I hadn't seen the series since I was a little kid of five or six years old, and really only remembered the cool SHADO interceptors, the purple-haired moonchix and the UFOs themselves. I didn't really recall the characters or how dark and grim the stories were... reading these books, written by British author John Burke - under the pseudonym, "Robert Miall" - based on the original scripts, really surprised me with their paranoid tone.
It wasn't until a decade or so later that I finally saw the show again during its run on the Sci-Fi Channel in the mid-90s, so for many years, these two books were my only exposure to Gerry Anderson's classic series.