Scales of Justice is the 18th in the Inspector Alleyn mystery series by Ngaio Marsh. This one was first published in 1955 and appears to be out of print. I found it at a Goodwill store. The characters aren't deep, I didn't care much about any of them, and the solution doesn't come as a shock; but it's a good enough book and fun to read. Not everything needs to be a masterpiece.
from google books:
Swevenings village is pretty as a picture, but its secrets are ugly; and its gentry dread the publication of Sir Harold Lacklander's memoirs. When one of them is murdered, Inspector Roderick Alleyn's investigation takes him through petty vendettas, an ex-commander's blend of whiskey and archery, and cocktails on the lawn with a femme fatale. But the motive he's angling for lies even deeper than the trout stream beneath the rustic bridge...Not surprisingly perhaps, reviews are few.