They also penned the remarkable Starchild Trilogy which was collected into an omnibus edition in 1977.
Cover of 1982 reprint.
On his own Williamson was aGrand Master, with a career that began in the earliest days of the genre and continued until just a few years ago. Not only did he write the famous Legion of Space tales in the 1930's, he also wrote on of the defining time travel novels, The Legion of Time (1938).
1967 paperback with cover by Jack Gaughan
Add to that his groundbreaking Seetee series, an early attempt to deal with anti-matter, or contraterrene matter as it was known them, and his Humanoids series which dealt with machine intelligence and you can see that Williamson left a legacy that's hard to match.
The radio show Dimension X even adapted one of his Humanoids stories, "With Folded Hands", which you can listen to online.