Elaine Hao, Program Manager of SharePoint Workflow @ Microsoft, put together a really nice series of blog posts on developing MOSS2007 workflows on the Microsoft SharePoint Products and Technologies Team Blog. At the end of the series, she attached a little mini whitepaper that covers everything she covered in her series. I found it extremely useful as I worked on some custom workflow for a client and decided to keep a copy of So You Want to Develop Custom SharePoint Workflows for myself.
This is way more helpful than aynihng else I’ve looked at.