In a layered architecture using Entity Framework, should I return POCO classes from the BLL? (Architecture guidance needed)?

You don't need to do anything in particular... as Mark said, there is no "right" answer. However, if your application is simple enough that you would simply be duplicating your classes (e.g. BookInfoUI & BookInfoBLL), then I'd recommend just using the business classes. The extra layer wouldn't serve a purpose, and so it shouldn't exist.

Eric Evans in DDD even recommends putting all your logic in the UI layer if you app is simple and has very little business logic.

What you're doing is basically the Repository pattern, for which Entity Framework and POCO are a great fit.

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