A survey on Web Content Filtering, written by the team that worked on the Spanish Filter at the project POESIA (Francisco Carrero, Enrique Puertas, Manuel de Buenaga and me), is included in the upcoming volume of the Series Advances In Computers. The full reference should be:
Gómez Hidalgo, J.M., Puertas Sánz, E., Carrero García, F., Buenaga Rodríguez, M. de. Web Content Filtering. In Marvin Zelkowitz (Ed.) Advances In Computers, 76, ISBN-13: 978-0-12-374811-9, Elsevier Academic Press, in press (expected Jun. 2009).