On decisionmodels, the website looks like it is coming straight from the 90s (but after all so does excel) but it is a gold mine of stuff to know before one can claims to really understand excel. I highly recommend the reading to anyone aspiring to be a "poweruser".
Seconded. decisionmodels.com has excellent stuff on the difference between F9, sh-F9 and ctrl-sh-F9, and explains when the formula graph gets rebuilt. When you're building calc heavy XLL addins it's important to have a clear understanding of the behaviour of the calc engine code invoking your addin.