This may be the third book in the series, but it was the fourth (or was it fifth?) book that I read. So really, order isn't that big a deal, unless you're deeply, deeply interested in the relationships.
By that I mean Libertus and his long lost wife, Marcus and his new wife (this was started in the previous book, A Pattern of Blood (link to review) ) and Junio and an unamed slave girl (also started in the previous book). Personally, I don't think it's that important, since apart from Libertus, all the romances aren't integral to the plot at all.
In this book, Libertus is called to investigate a politically sensitive mystery. Felix, a favourite of the emperor, dies suddenly. At first, everyone thinks it's an accident, but with one guest who came under false pretenses, and who is conveniently missing (there's another missing guest too), there's definitely more to this case than it seems.
There was a twist to this case that I definitely didn't expect. In fact, I just realised that Libertus is very much like Poirot - he keeps his cards to his chest, and then he reveals everything at one go. The only difference is that the book is told through the eyes of Libertus and not his sidekick (like how Poirot's mysteries are mainly told through the eyes of Hastings or whoever is his sidekick).
It's an enjoyable book, though it's very much like the previous books in the series (actually, it's probably the similar in style to books 1 to 4)