Deceiving the Duke of Kerrington by Ginny Hartman is based on the trusted plot of a charade where people resembling each other impersonate the other. As such the book is an amusing read; when the plot becomes too obvious, you still want to read on to see how on earth the author is going to get herself out of the pit she is digging for herself page after page, deeper and steeper.
Are you looking for a sex, crime, and murder mystery set in Regency England? I'm afraid you haven't found it. All that dates the book's story is the mention of 1819 at the beginning of the first chapter. After that, it just is a generic sex and crime story. Stone Devil Duke by K. J. Jackson is showing up major defects in writing without doing at least some minimal research.
The Stolen Kingdom by Ross Rosenfeld is a very unhappy book, or maybe an unfortunate one. It definitely suffers from multiple personae disorder. It tells the story of the missing prince in a new way; the story is placed in an imaginary Europe without giving away any cultural details as to where exactly it is.