Andre Norton was one of SFF's most prolific writers, publishing a huge number of books (I gave up trying to count the list on Wikipedia, but the total must be in the region of 200) between 1934 and her death in 2005. Many of these were co-authored with various other female novelists. The really strange thing is that I have read hardly any of her books, despite hoovering up all the SFF I could find in the 1960s and 70s. I can only assume that her work was not stocked in the libraries I used for much of that time, and I might also have been perplexed about where to start when faced with rows of her books in a bookshop.
The only exception I can recall is Judgment on Janus, which I read in the 1960s and enjoyed so much that I went looking for a copy a couple of decades later, with the bonus of finding the sequel (Victory on Janus) at the same time. I'll re-read and review those another time.
The X Factor, published in 1965, was recommended to me as a book I might like by someone on an SFF forum, so I tracked down a copy. The setting is a far future in which humanity has spread across many star systems, encountering various alien races. The hero is Diskan Fentress, a young man handicapped by a huge, clumsy body and a slow mind, but with an unusual ability to connect mentally with animals. Despairing of ever fitting in with his quick and graceful contemporaries or of living up to his famous space explorer father, he steals a spaceship and crash-lands on a planet found by his father which was identified as habitable but to be avoided. There he is found by an intelligent and telepathic but non-technical alien race, who see in Fentress an opportunity to achieve their own mysterious aims. Archaeologists and treasure hunters complicate matters and the hero battles though many dangers and hardships before the ambiguous and rather mystical conclusion.
Typical of the period, this is a short and fast-moving adventure story which I devoured in a couple of sessions. Characterisation is rather better than usual because of the focus on Fentress and the way in which he develops during the story. The aliens are also interesting, their strange viewpoints being glimpsed occasionally. Not a classic work, but well worth the time to read.