I've been using excel for a long time, but my new job requires me to be more
of a power user than I currently am.

Any suggestions on online course/tutorials? Books? Free websites that can
assist in my learning?

I'm comfortable with using most excel functions, but need to improve most in
the advanced functions area, better/efficient workbook design, preparing a
workbook for distribution/collaboration, and generally those tips and tricks
that help a user go from good to great.

Any ideas are much appreciated.