The authors' best advice regarding trees, incidentally, is "SQL still lacks a truly adequate, scaleable[sic] processing of tree structures." It's hard to disagree with that conclusion, and it's hard to argue the authors' decision - in light of that conclusion - to not fully address tree structures in SQL. Out of curiosity I'll probably take a look at Joe Celko's Trees and Hierarchies in SQL for Smarties soon, but to be honest I think professionally my solution is going to be "stay the hell away."
However, in general the book covers a lot of ground in much better depth than most books - offering interesting examples, SQL demonstrations of principles, and performance comparisons of different approaches that is well worth reading, and keeping handy as a reference. It is, frankly, the only database book I've read thoroughly and still been satisfied enough with to recommend.