E pub is a popular format used in Nook, iBook and other readers. Barnes Noble produce their e books in this format. Nook is a free downloadable file. If Nook does not work, Adobe Digital Edition (ADE) is a free downloaded book reader that always seems to works well for e pub books
E mobi is used by Amazon in their kindle format. Kindle is a free downloadable file.
PDF is the most versatile file in that every computer has Adobe Acrobat, which is all that is necessary to read this file. However, it is not as user-friendly as the other two formats
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