Barbara Monajem 
Lady Rosamund and the Poison Pen [EPUB ebook] 
A Rosie and McBrae Mystery

Lady Rosamund Phipps, daughter of an earl, has a secret. Well, more than one. Such as the fact that she’s so uninterested in sex that she married a man who promised to leave her alone and stick to his mistress. And a secret only her family knows—the mortifying compulsion to check things over and over. Society condemns people like her to asylums. But when she discovers the dead body of a footman on the stairs, everything she’s tried to hide for years may be spilled out in broad daylight.

First the anonymous caricaturist, Corvus, implicates Lady Rosamund in a series of scandalous prints. Worse, though, are the poison pen letters that indicate someone knows the shameful secret of her compulsions. She cannot do detective work on her own without seeming odder than she already is, but she has no choice if she is to unmask both Corvus and the poison pen.