From the back cover of How do I do that in PostGIS:
For those who are unsure if SQL is a sufficient language for performing GIS tasks, this book is for you. This guide follows the topic headings from the book How do I do that in ArcGIS/Manifold, as a way to illustrate the capabilities of the PostGIS SQL engine for accomplishing classic GIS tasks. With this book as a resource, users will be able to perform many classic GIS functions using nothing but SQL.
I was really impressed with just how many of the tasks defined in the original How do I do that in ArcGIS/Manifold could be accomplished using just using SQL.
The nice thing about these books is that since they follow the tasks outlined in How do I do that in ArcGIS/Manifold, they cover a fairly comprehensive set of GIS tasks. So, I think this will be a great reference manual for you as you make use of SQL for performing GIS. In fact, I found that as I was writing the book, I began referring back to it to help me better understand how to write the queries – so, I guess you could say that I was my first satisfied customer!