Murder of a Dead Man by Holly Newman

As Lady Cecilia Branstoke convalesces after a severe bout of influenza, an urgent plea for assistance arrives from their friend, Mrs. Montgomery: The Earl of Soothcoor languishes in a goal in Lincolnshire, accused of murder. The victim? Mrs. Montgomery’s husband, Malcolm, reported dead over two years ago!
The Branstokes travel to Camden House Sanatorium in Lincolnshire where Mr. Montgomery had secretly been a patient. In society, Camden House Sanatorium had a reputation as one of the most modern and successful sanatoriums, a model for other institutions.

But can a reputation be too perfect?
To investigate, Lady Branstoke registers at Camden House as a patient needing further care to recover from her influenza. Meanwhile, Sir James probes outside connections in pursuit of establishing Soothcoor’s innocence and identifying the actual murderer.
As the Branstokes delve into the tangled web surrounding Mr. Montgomery’s life and death, the façade of Camden House as the perfect sanatorium cracks, revealing the canker of disintegration beneath the polished exterior.