I bought an e-book (Mobi) from another source. I got it on my Kindle 2nd generation, but I don't usually read on that device. How do I get it to sync to other devices? It does not show up on my Kindle content on Amazon. I don't understand why.
If you bought it from another source, it would not show up under Books in your MYCD. It would show up under Docs and ONLY if you used email or Send to Kindle and specified to keep a copy in the cloud. On other devices (depending on what they are), it may also not appear under Books but under Docs. Non-Amazon books are not usually kept with Amazon books.
If one or more of the devices is Kindle for PC or Kindle for Mac, you must MANUALLY place the book on those devices. You can "deliver" to other devices from the cloud but not the app for the personal computers (whether Windows or Apple).
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There is no such thing as a stupid question!
Corollary -- so long as you read before you post and stay on topic. :)