I can successfully send other books to my Kindle via the "Send Ebook" button within my account, and "Learning Agile" is available in .mobi format. For this particular book the only options I get when clicking on the "Send Ebook" button are to send to Dropbox or Google Drive.
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The reason there isn't a Send to Kindle button for that ebook is that the mobi file is over the 50mb size limit that Amazon sets for that service. For mobi files larger than 50mb, you'll have to use an alternate method for getting the mobi on your Kindle, such as downloading it directly from your oreilly.com account to your Kindle, or plugging the Kindle into your computer and copying the mobi file to the Documents folder on the Kindle.
O'Reilly Book Support