Mary McMullen [1920-1986] was a pseudonym of Mary Reilly Wilson, mystery and suspense writer, daughter of mystery writer Helen Reilly and sister of mystery writer Ursula Curliss
A disgruntled employee who was refused a raise began to wreak havoc by spying on his workmates and keeping these little notes. Who would have ever thought that colleagues could be so murderous? Frame ups, attempted murder, hired assassins. The writing and characterization were solid as ever but it all turned to be just a tad too implausible in the end.
I collected loads of this Keyhole Crime series when I was young. They are numbered and take up a lot of shelf room, albeit they now live on a shelf in the attic. Many times over the years I have been tempted to just give the whole lot away but I'm glad I kept them, especially as I see they have become a bit of a collectors item. One day I will treck up to the attic and look at the summaries on the backs, as that is the only way I will remember what each one was about, and log them on Goodreads. Most of them are so old that no summary is offered on Amazon etc.