[GeoffR]

If you buy the from the device's bookstore instead of from the website, then the book will download immediately after purchase without doing a full sync.

If you buy from the website, then you can search for the book you bought in the device's bookstore and download it without doing a full sync, but it is probably faster just to sync.

Edit: You can also do a full sync, but then tap the sync tile again to cancel the rest of the sync once the books you bought have downloaded.