Packt Publishing is looking to expand their already extensive library of open source books. Their recently published road map for 2009 includes eZ Publish Templates, Customizing Zope Sites, Apache Jackrabbit, Apache CXF, Openbravo, OpenEMM, Django e-commerce, Bazaar VCS, and Groovy dsl.
If you know these technologies and are interested in writing a book, you should email Packt at email@example.com. From what I hear from the authors that I know, Packt is a good company to work with and their royalties (roughly 11.25% of the cover price) are generous. Packt also contributes a significant amount of money to the projects themselves through their royalty scheme and their annual Open Source CMS Award.