a) How Browsers Work – an in-depth look about browser internals.
b) From the same author, client side performance resources and tools – at http://taligarsiel.com/ClientSidePerformance.html.
Part 1 – “Understanding and measuring Performance”
Part 2 – “Loadtime”
Part 3 – “Runtime”
Part 4 – “Coping in the User Interface”
Includes a cross-browser profiling tool – “The DOM Monster” (have not tried it yet)