A big change of pace here, with a detective story in place of the usual adventure. Although I'm not a fan of this particular genre, and am thus unable to say how this compares with, say, Agatha Christie, the mystery seems cleverly plotted and I for one didn't guess the ending beforehand!
Unfortunately Leslie Wood, who contributed the bold cover paintings for the earlier novels, seemed to have moved on and from now on the cover illustrations for the books became increasingly poor.