Fowler discusses a concept he's calling a Request Stream Map (based on a co-worker's work). Interesting idea for how to break down what the pieces are, and the flow of, handling a request into a system. Really clarifies what is involved in crossing the various layers and tiers of a system so you can focus on the cost/benefit of each layer and tier
Very even-handed coverage (IMHO) of the current tension in system design between when to use an RDBMS and when to look at alternative data storage models