Jim Cheshire 
Creating Microsoft Expression Web 4 Add-ins [PDF ebook] 
Using Existing HTML and JavaScript Skills to Build Add-ins for Microsoft Expression Web

With the release of Expression Web 4, Microsoft moved to a new add-in model that allows for the development using HTML and Java Script. This enables web designers to use existing skills to develop add-ins and extend the functionality of Expression Web. This book serves as a focused guide to creating add-ins in Expression Web 4. It will walk you through the development and testing of a full-featured add-in for Expression Web 4. If you use Expression Web 4 and want to extend the functionality of the product, either for your own design needs or because you want to market add-ins to other Expression Web users, then Creating Microsoft Expression Web 4 Add-Ins is perfect for you!