I have been using Dropbox to archive e-books purchased directly from publishers like Apress, O’Reilly, Packt, and so on. I just realized that I can quite easily send these books (in formats supported by the Kindle) to my Amazon Kindle library. Amazon has a page that details the different options available to send content to the Kindle library. The Kindle library has a total limit of 5 GB for personal documents.
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Source:
http://delog.wordpress.com/2013/01/09/send-content-to-the-amazon-kindle-library/