Yes, ADE is free - that is the transfer software to get your books onto your reader.
As said, it is free.
As well as transfer, it can also be used for standalone reading of ebooks on the PC.
There are several PC readers for non-DRM ebooks.
ADE can read Adobe DRM ebooks, which most can't. (Sony Reader can).
.. but ADE won't transfer library books to PRS-350.