Late August, AD 79, a few days before the eruption of Vesuvius: Aquaduct engineer Marcus Attilius Primus comes to the Bay of Neapolis to take over the maintainance of the Aqua Augusta. Not only does he have a mysterious breakage in the aquaduct to deal with, but he's also got to work out what happened to his predecessor.
The intricately-detailed setting meant I enjoyed this rather more than "Enigma" or "Archangel". My only complaint was that the ending felt a bit contrived -- although it could have been much worse, and the reader's sense of impending doom limits what can be done with the plot! Recommended, anyway, particularly if you find engineering interesting.
ISBN 0099282615.