I'm learning about workflow in SharePoint 2013 at the moment and when looking for some background reading, I came across this series of videos. They cover the underlying WF platform and development best practices.
Workflow and Custom Activities
Source code for this is here: http://archive.msdn.microsoft.com/wf4BP
The Workflow developer centre on MSDN: http://msdn.microsoft.com/en-us/vstudio/jj684582
Some of these videos are posed on the endpoint.tv channel (http://channel9.msdn.com/Shows/Endpoint) but no new content is being posted there as the AppFabric.tv channel (http://channel9.msdn.com/Shows/AppFabric-tv) seems to be the new focus. This bills itself as covering "the latest in Windows Azure AppFabric, Windows Server AppFabric, Windows Communication Foundation (WCF) and Windows Workflow Foundation (WF4)". Still there has been no new posts there either for over a year but posts are still going to the Workflow TV channel (http://channel9.msdn.com/Shows/Workflow-TV).