A few years ago, my career touched on Sharepoint 2003 but I haven't worked with it in some time. My group is starting to see some traction with it so I decided to update my understanding.
First off, as Microsoft is fond of doing, there are at least five different versions of Sharepoint. This chart, while far from concise, communicates the functionality in the different offerings. I found it helpful for my breath of understanding.
Also, I'm investigating books I can read in my free time. Here's what I came up with along with some suggestions from the gurus at my firm (thanks, Leon, Steve):
Microsoft SharePoint 2007 Unleashed
Professional SharePoint 2007 Development
Pro SharePoint 2007 Development Techniques (Pro)
Inside Microsoft Windows SharePoint Services 3.0 (Pro Developer)
Real World SharePoint 2007
Microsoft SharePoint: Building Office 2007 Solutions in C# 2005
Microsoft SharePoint 2007 Technologies: Planning, Design and Implementation
Professional SharePoint 2007 Web Content Management Development
Update (1/14/2008): Good whitepaper from Chappell. It's a bit old but the concepts hold true.
Update (1/15/2008): 100-level but clear and concise roadmap from Microsoft.
Update (1/17/2008): I ended up buying Professional SharePoint 2007 Development...I'll post my thoughts.