My colleague Aidan Garnish has written a great review of the Scott Hillier's book, Microsoft SharePoint: Building Office 2007 Solutions in C# 2005.  It appears that this is a great book to get a good overview and grounding in the main concepts relating to Application Development using Microsoft Office SharePoint Server 2007.  Check out Aidan's review at http://aidangarnish.typepad.com/aidangarnish/2007/08/book-review---m.html and you can get the book at Computer Manuals - http://www.compman.co.uk/scripts/browse.asp?ref=814091

Aidan's blog is a great resource too for anyone getting into developing on MOSS2007.  Aidan has a lot of experience with SharePoint 2003 and is now along with the rest of the team working with MOSS 2007 to develop Internet Content Managed Sites.