Microsoft has a good amount of downloadable content for SharePoint.  They make some pretty good references as all of the content is actually available on TechNet.  I like to keep these on an external drive however, In case i need to research something but don't have Internet access at a client's site.

http://technet.microsoft.com/en-us/library/cc262788.aspx

The titles are:

• Getting started with Office SharePoint Server 2007
• Planning and architecture for Office SharePoint Server 2007, part 1
• Planning and architecture for Office SharePoint Server 2007, part 2
• Governance guide for Office SharePoint Server 2007
• Planning an extranet environment for Office SharePoint Server 2007
• Deliver accessible solutions (white paper)
• Best practices for developing accessible Web sites (white paper)
• Deployment for Office SharePoint Server 2007
• Upgrading to Office SharePoint Server 2007
• Deploying Office SharePoint Server 2007 Service Pack 1
• Installation guide for Office SharePoint Server 2007
• Design and build sites for Office SharePoint Server 2007
• Administering enterprise search for Office SharePoint Server 2007
• Web publishing and planning guide for Office SharePoint Server 2007
• Records management guide for Office SharePoint Server 2007
• Document management guide for Office SharePoint Server 2007