There's a bunch of books listed here:
Slipstick Systems Bookstore - Developer Resources:
http://www.slipstick.com/books/dev.htm
"Microsoft Outlook Programming - Jumpstart for Administrators, Developers,
and Power Users" is a great book for new Outlook programmers.
Also see
http://www.outlookcode.com/.
You can program Outlook with the built-in VBA engine that uses macros like
other Office applications, develop COM Add-Ins with Visual Studio 6.0, 2003,
2005, or automate Outlook's Object Model with any programming language that
can use COM interfaces.
--
Eric Legault - Outlook MVP, MCDBA, MCTS (SharePoint programming, etc.)
Try Picture Attachments Wizard for Outlook:
http://www.collaborativeinnovations.ca
Blog:
http://blogs.officezealot.com/legault/