I've just been doing some reading around the area of open content and the EPUB format and I want to make some observations in relation to these and Personal Learning Environments (PLEs).
To me a PLE is the collection of tools and resources that a learner develops over time and carries forward through their learning journey, possibly life-long. Each learner will have a different PLE based opon the tools that they have chosen that work for them. The resources they collect are then influenced by those tools.