If you’re interested in Excel services (as opposed to straight up Excel), there are a number of online training courses at:
http://blogs.msdn.com/excel/archive/2008/09/09/new-online-training-courses-for-excel-services.aspx
Excel services allows you to work with Excel sheets on a server without Excel locally installed and with server configured permissions. It’s going to be an important part of a Sharepoint deployment but there is a lot of confusion around what Excel services is and what it can do. Hopefully these training classes can help clear things up.