1) Brett's original article, from the year 2000, with a very useful diagram... which had actually been posted on SearchEngineWorld, WebmasterWorld's "sister" site, which Brett shut down recently. Much of the content on that site hasn't yet been redirected. Here's the old link...

Analysis and Dissection of the Theme Based Search Engine
[searchengineworld.com...]

2) the article by ciml, the co-moderator of this SEM Research Topics forum, originally posted in May, 2002, which updates the concept and brings in the question of PageRank distribution. It refers to Brett's diagram, but you can probably get something out of it without that...

Search Engine Theme Pyramids and Google
Optimising the Pyramid for PageRank
[webmasterworld.com...]