It's not out yet, but there is a promising free ebook reader on the horizon.
dotReader from OSoft. It is being based on the OpenReader Consortium's open-source XML format, and is going to be capable of working in a variety of formats and includes the capability to add more through plugins. Currently, they are planning to release software in August. No idea yet how soon a WM version will become available, but they are actively developing for PDAs as well as other platforms.