I think you can glean the history from this site:
http://pocketbook-free.sourceforge.net/en/index.shtml. Looking at the sourceforge site that contains the downloads of SDKs and apps
http://sourceforge.net/projects/pocketbook-free/ shows that the oldest versions match up with the early 2009 announcement on the first site.
The sites also show that no new SDKs have been released in 3 years, while the devices keep changing. It's getting harder and harder to develop for new devices.