Mouse Training has released its training manuals and Quick Reference Guides for Office 2007, Office 2003, Office XP (2002) and Office 2000 for free. The materials include Word, Excel, Outlook, PowerPoint, Access, Project and Visio. All of the documents are in pdf format and can be downloaded here: http://www.mousetraining.co.uk/ms-office-training-manuals.html.
[UPD.] It seems the link above is not working. However you can download free books about Microsoft here.