In the mid-50s, there was some major changes going on in the publishing industry: readership for pulps and magazines were declining, and it was rising for novels. Authors had an interesting way to respond to this: take a couple of existing stories, rewrite bits and package them as a novel. A.E. van Vogt actually coined a term for this: We call it a ‘fix-up’ novel.
Go read A.E. van Vogt and the Fix-Up novel over on the Kirkus Reviews Blog.