Publishers can choose specific reading options for each book, and there are a number of reasons the Kindle option might not be enabled for a particular book.
Especially if the file is heavily designed, illustrated, or has a lot of photographs, the file will often not render well on Kindle devices. The file size may also be too large to send to the Kindle.
If the Send to Kindle button is not available for a particular book, we encourage you to download to another device, app, or your computer. For a full list of devices we support, see here.