I just recently began my journey down the road of learning about WSS and MOSS. My primary focus of course is for the workflow portion of the product, but what I have found while building that base of knowledge, is that you do need to know quite a bit about SharePoint just to be able to understand what you can do with custom workflows.
I picked up this book written by Ted Pattison and Daniel Larson and I've been very impressed with the content, the level of detail and code samples. I can see that this is a book I am going to reference again and again. I was actually working with a client who had WSS questions the week I purchased this book and the answers were right there in front of me. Talk about good timing!
This book is definitely for the developer but it does have topics and admin might be interested in like SharePoint architecture, deployment and application security. And of course the chapter on workflow was most excellent…the text was very clear at explaining several of the properties and fields that are critical pieces of custom workflows.
I'm not sure what those folks who have lots of experience with WSS think of the book, but for me, it was a good purchase.