The Unpleasant Profession of Jonathan Hoag

Every evening, Jonathan Hoag finds a mysterious reddish substance under his fingernails, with no memory of how it got there. He hires the husband-and-wife detective team of Ted and Cynthia Randall to follow him and find out. But Ted and Cynthia are mystified when they find that their own memories of what happens during their investigation do not match. There is a thirteenth floor to Jonathan's building that does not exist, there are mysterious and threatening beings living inside mirrors, and all of reality is not what they thought it was.

Contents
...And He Built a Crooked House... (1941)
They (1941)
The Unpleasant Profession of Jonathan Hoag (1942)
Our Fair City (1949)
The Man Who Traveled in Elephants (1957)
...All You Zombies... (1959)