There is a fairly detailed section on monitoring web applications using Nagios and Sahi in Gerhard Lausser's new book (in German): "Nagios - The Practice Book: Open Source Monitoring in the Enterprise", ISBN: 978-3-8273-2800-7
http://www.amazon.de/Nagios-Praxisbuch-Open-Source-Monitoring-Unternehmen/dp/3827328004
