I am not a speed reader, but I do read a lot of books. I get content for my Kindle app from several different places, including Amazon.com.
Content (as long as it is in a format used by the Kindle app) can be sent to Amazon.com and registered in their cloud storage for free. I've been a member of Amazon since 2012 and have sent over 2,500 different. Mobi,. Pdf and. Epub files to be stored in the Kindle cloud.
Unfortunately, today I found out that Amazon/Kindle can't find much of the content I've sent/registered with them. Amazon insists all my content is still registered and available but can only find 357 different files. (This "lost" content is the reason Amazon.com doesn't get 5 stars from me.)