What in-depth spatial database systems tutorials exist?

While the PostGIS in Action book isn't a step by step tutorial, it is a very handy guide, and I recommend it to anyone who wants to truly understand PostGIS and get beyond just the basics of PostGIS.

you could try http://www.bostongis.com/ they have a number of guides for PostGIS also the links provided by the other posters are also excellent tutorials. For general GIS information, ESRI's website has some excellant tutorials www.esri.com. and also provide training video's in relation to their software. Other links to try http://geospatial.ucdenver.edu/foss4g/resources/webinars

Perhaps the Intro to PostGIS would fit the bill, http://postgis.net/workshops/postgis-intro/