I Learned a lot from "Tcl for Web Nerds" and its companion "SQL for web nerds" (all links listed here: http://openacs.org/doc/ ). Back when I started Tcl, I was more like a emacs-lisp script kiddie. Because Tcl was so easy to learn and because OpenACS was such a great MVC style framework, it didn't take long to master the framework itself and dive into the interesting things that were related more with the architecture of a web-app and not just the syntax of a programming language.
The "whereas" poem, as I like to call it, that announced Miguel Sofer's inclusion in the Tcl Core team was real fun to read too - http://code.activestate.com/lists/tcl-core/1983/
Miguel Sofer had described an algorithm for representing hierarchical data in a RDBMS. What he put forward could be thought of as a different kind of a nested set representation. Whereas the nested-set approach involved keeping track of two numeric values (left & right) for each node in our relational records based tree, Miguel Sofer's algorithm would use the ability to lexically sort a base159 encoded string. This way, tree operations could be implemented via sub-string matching and sorting.
His algorithm was implemented in OpenACS because it allowed an efficient implementation of the OpenACS nodes table. Each URI in OpenACS has a node record associated with it and all these nodes are hierarchical records - http://openacs.org/forums/message-view?message_id=16799
The OpenACS nodes system allows the implementation of a fine grained permissions system which enables a child node to auto-magically inherit the permissions of its parent node - this is, if the child node didn't have any specific permissions set on itself - http://openacs.org/doc/permissions-tediously-explained.html
When I was learning ruby I implemented this algorithm using active-record - https://github.com/gautamc/hierarchical_objects The utdt.edu link hosting the pdf that Miguel Sofer created is broken now; I found a copy here: http://www.tetilab.com/roberto/pgsql/postgres-trees.pdf