[link|http://www.intelligententerprise.com/001020/celko.jhtml?_requestid=1126911|Some answers to some common questions about SQL trees and hierarchies]
A surprising one: [link|http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnsqlpro03/html/sql03b8.asp|Trees in SQL Server] at MSDN.
[link|http://www.dbdebunk.com/publications.html|PRACTICAL DATABASE FOUNDATIONS]
[link|http://www.google.com/search?q=Trees+in+SQL|Google for: Trees in SQL]