Vitruvius, de architectura, and Frontinius commentarius de acquis. Frontinius also wrote the oldest roman tract we have on land -surveying, from what i understand the reconstruction of the manuscript was important for the history of philology (Lachmann!).
Vitruv's online in original and English translation at the Perseus project <http://www.perseus.tufts.edu/>. Mirror sites at oxford and berlin <http://perseus.mpiwg-berlin.mpg.de/>, <http://perseus.csad.ox.ac.uk>.
Anita