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Too many users:
Some eBooks have restrictions on the number of simultaneous users; try accessing the eBook again in a few minutes.
eBooks/collections with limited users:
Most eReaders can read PDFs, epub or azw book formats.
pdf, Kindle format (azw), txt, mp3, aa, aax. Also can support doc(x), html, rtf, jpeg, gif, png, bmp, pdf creation (with a for-fee conversion service from Amazon). No epub.
ePub (including non or Adobe DRM), pdf, xls(x), doc(s), ppt(x), pps, txt, docm, xlsm, pptm, ppsx, ppsm, jpg, gif, png, bmp, mp3, mp4, aac
Most formats, depending on downloaded app