A very interesting concept for a book: we already know what the ending is going to be, we just want to know how we get there. Similar to the film, Titanic; you know the ship is going to sink, let's see how this young love is going to cope with it.
The entire book, quite rightly, I suppose, builds up slowly to "The Eruption". Marcus Attilus, in charge of Aqueduct Maintenance, is going to fix a blockage and takes some crew with him. His crew are a mixed bunch, to say the least, but Harris reassures the reader that despite some petty faults, these are all good men, all good men who are going to get smoked!
There's a love interest, there's a case of a missing predecessor, some brutality from the higher classes, some corrupt people and a cameo from the great Pliny. Lots of strands of story, most quite under-developed; but that is the entire point.
Harris knows the reader will enjoy the little sub-plots and so on, but you're not buying the book to see if Attilus gets the girl, are you? No. You're buying it to read about Pompeii getting liquidised. And that is the last third of the book, and boy-oh-boy, Harris does a magical job writing it!
You'll also like the snippets of volcano fact he inserts at the start of each chapter.