For me, my library index file got corrupted somehow. For whatever reason, one particular book, when loaded into the library, would corrupt the index file. I had to clear the library (do not delete from your PC!) and then delete the index file and reload my library, with the exception of the offending book. I found which it was through trial and error (it was some book I had converted to epub on my own, not a purchased book). I can't remember the name of the index file, but if you uninstall the Reader software, and then delete the files under your Users folder, you should be fine.
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