The Lurker at the Threshold

August Derleth’s “Posthumous Collaborations”
After Lovecraft’s death, August Derleth based a number of stories on fragments written by Lovecraft and published these as “posthumous collaborations” between Lovecraft and Derleth. Since then, these stories have been marketed as being by “H.P. Lovecraft and August Derleth”, “H.P. Lovecraft with August Derleth”, or by H.P. Lovecraft alone.
Some of the books that make this error include:

• The Lurker at the Threshold (Arkham House, 1945; Beagle/Ballantine Books, 1971; Carroll & Graf, 1988)
• The Survivor and Others (Arkham House, 1957; Ballantine, 1962)
• The Shuttered Room and Other Tales of Terror (Beagle/Ballantine Books, 1971)
• The Watchers Out of Time and Others (Arkham House, 1974)
• The Watchers Out of Time (Carroll & Graf, 1991; Del Rey, 2008)

The following excerpt from S.T. Joshi’s H.P. Lovecraft: A Comprehensive Bibliography explains the matter further:     
These sixteen stories, listed as by “H.P. Lovecraft and August Derleth,” were in fact written almost entirely by Derleth. In most cases, the stories were based on one or more ideas noted in Lovecraft’s Commonplace Book; for example, “The Fisherman of Falcon Point” was based on this entry: “Fisherman casts his net into the sea by moonlight—what he finds.” Plotting, description, dialogue, characterization, and other elements were entirely by Derleth. As such they cannot be classified as works by Lovecraft.