Death in the Dark

"The truth can't hurt anyone," she said rather faintly.
He did not speak for a minute. Then he said without looking at her. "Not if he's innocent."
David Merle, a young circus acrobat, is arrested and convicted of the murder in London of wealthy eccentric Joshua Fallowes. Only his sister, Judy, and their aunt really believe in his innocence, although kindly Ben Levy stands by Judy (with whom he is rather smitten).
Things are looking bad for David until his sister gets a note from thirteen-year-old Toby (last seen in The Mystery of the Kneeling Woman). He has discovered some points of interest concerning the case, and this information sends Judy off to Sard Manor, a run-down country mansion complete with a privately-run zoo. She sets to work, sleuthing as an undercover housemaid. Fortunately Judy has a loyal friend in Toby, who comes furnished with a heroic Scotland Yard stepfather-none other than Inspector Collier. She'll need all the help she can get, when the potential antagonists include not only the denizens of the Manor, but the beasts which lurk without.
Death in the Dark was first published in 1938. This new edition includes an introduction by crime fiction historian Curtis Evans.