When I was in junior high school, I joined the Science Fiction Book Club. One of the books I got from the club was an anthology that included several stories by Fredric Brown (who was primarily a mystery writer but occasionally delved into science fiction). Some of Brown's stories in the anthology were a mere page or two, and I loved their humor and surprise endings. As soon as I could, I went to the Boulder Public Library to load up on as much Brown as I could find. It turned out the library had just two of his science fiction novels: Martians, Go Home (1955), and What Mad Universe (1949). They were both terrific.
In Martians, Go Home a race of cartoonish little green men invade Earth for the sole purpose of being hideously bothersome pests, behaving very much like Internet trolls and Second Life griefers. (Artist Kelly Freas perfectly captured the personality of the martians in his cover painting for Astounding Science Fiction.) In What Mad Universe a man gets thrown into a parallel universe and has to figure out how to get back home. Both books are semi-parodies of science fiction novels (the protagonists in each novel are science fiction writers), with plenty of Brown's signature wry humor. If you've not read these novels, I highly recommend them both.
It wasn't until I was in high school that I scored a copy of The Mind Thing (1961), which is probably my favorite Brown novel, even though it is not as well-known as the other two novels, and could be arguably be classified a horror novel. The Mind Thing is an alien being (which looks like a turtle shell) that has been banished to Earth for committing crimes on its home planet. It is unable to move on its own, but can hijack the nervous system of any sleeping animal within range and take control of its mind and body. To leave the body, it forces the host to commit suicide. The alien goes on a spree, hopping into people's bodies and killing them, as it moves forward with a plan to make the Earth ripe for takeover (in the hope that its fellow creatures will forgive its past crimes and hail it a hero). Eventually, a smart fellow (an MIT professor on vacation) figures out what's going on and takes it upon himself to save the planet from the evil space alien.
Long of of print, all three are available in Kindle editions. (I don't recommend Rogue in Space or The Lights in the Sky are Stars because they both stink, unfortunately.)