A colleague asked me if multiple Kindles could share the same booked purchased from the Kindle store. Rather than email a reply I made this post since others may be interested in the reply.
From the Kindle support pages we see the FAQ ‘Can I share content with other Kindles?’ to which the answer is:
Downloading to Multiple Devices
Content purchased from the Kindle Store can be downloaded to your Kindle, iPhone, or iPod touch as long as you’ve registered the device to the Amazon.com account that purchased the Kindle content. There is no limit on the number of times a title can be downloaded to a registered device, but there may be limits on the number of devices (usually 6) that can simultaneously use a single book.
That means you can download and read your books on any Kindle device you own as long you’ve registered each device to the Amazon.com account where your Kindle library is stored.
You can see the items in your Kindle library under Archived Items on your Kindle and send downloads to your registered Kindles from the “Your Orders” section of the Manage Your Kindle page.
So the simple answer is up to 6 devices registered to the same Kindle account can share a Kindle book edition at one time. This also includes the other Kindle reading devices such as PCs, Macs, iPhone/iPad and Blackberry. The single, shared account is key. However, if you delete a book from one of the devices it should free up a slot for another device.
