L. C. Tyler, Herring on the Nile [2011 (UK); 2012 (US)]
Felony and Mayhem Press (US)
Available in print and as an ebook
This is the fourth in the series, and (to date) the second best. (The first--The Herring Seller's Apprentice--seems to me slightly better.) Following the events in The Herring in the Library, crime writer Ethelred Tressider decides to take a Nile cruise (for research purposes...but he also intended to have company). Things don't work out exactly as planned, as his expected company does not accompany him--but his agent, Elsie Thirkettle, does.
Ethelred is cornered by another passender on the boat, a Mr. Purbright, who alleges himself to be an agent of M.I.6, and, mistaking Ethelred (one of whose pen names is Peter Fielding) for Paul Fielder, a best-selling thriller writer' he's seeking help in dealing with possible terrorists and needs some backup. The other passengers on the boat are the usual strangely assorted lot, none of whom may be quite what they seem to be. (Among the passengers is Herbie Proctor, a somewhat sleazy and incompetent PI there to protect a client he has never met. We have encountered Herbie before.)
Not too surprisingly, death intrudes, and Ethelred and Elsie (more or less independently) try to determine what's going on. For the reader, identifying the murderer should be fairly simple...but figuring out the motive is somewhat less simple.
Be on the lookout for a mention of a famous mystery author (other than Agatha Christie, whose name pops up quite often) and one of that author's books. Also, treat yourself to Ethelred's responses to a set of interview requests (questions to be answered by email) from various newspapers in England.
A very good entry in what is, so far, a very satisfying series.