'Envious Casca' was first published in 1941 and is the second in the Inspector Hemingway series. As well as her many regency novels, Heyer wrote 12 detective novels, including four featuring Inspector Hemingway and four with Superintendent Hannasyde.
Synopsis from amazon.co.uk: "It is no ordinary Christmas at Lexham Manor. Six holiday guests find themselves the suspects of a murder enquiry when the old Scrooge, Nathaniel Herriad, who owns the substantial estate, is found stabbed in the back. Whilst the delicate matter of inheritance could be the key to this crime, the real conundrum is how any of the suspects could have entered the locked room to commit this foul deed. For Inspector Hemingway of Scotland Yard, 'tis the season to find whodunit."
A fan based website for Georgette Heyer is here. Isn't the cover delightful?