Some links that have proven helpful thus far:
Bernard Embden — Bernard has owned a 1978 XJ-S since 1980. His site chronicles the changes he made to the car over that time. Many pictures, charts, and explanations are provided for your convenience.
Kirby Palm — Kirby documents his experience in a 700 page MS Word document. Just the little I have read was incredibly helpful. Any question about any possible modification seems to have been answered in this book.
Sean's Tech Pages — Sean includes a Jaguar VIN decoder, vacuum routing diagrams, and a list of helpful tools. There is a long page of other links as well.
Manual Transmission Conversion — Here's a nice idea. This company sells manual transmissions to replace the 3 speed automatic which most Jags had. They claim to increase acceleration, top speed, and most importantly ... fuel economy!