You can't move anything to the ext SD card. The system does in automatically in the case of the Kindle app and in all other cases using the "Move to sd card" method described above.
As far as I know there is no way to get the Kindle app to look for books in a folder on the ext SD card. Some reader apps do have that option. What I was referring to was looking at the Amazon Kindle app detail in "settings" > "Application Manager" > "Downloaded" list then tap "Amazon Kindle" to display the detail. Under the "Storage" section of the detail it shows the SD card app being 0 meaning that non of the app is on the ext SD card. My Tab shows total Data of 66.42 MB and SD card data of 53.25 MB meaning that 53.25 MB of the app data (books) is on the ext SD card. I cannot locate the folder(s) on the ext SD card where any of the app and data files are stored using any file manager. The OS has hid them from view. All the Kindle books are initially stored in the Kindle Files folder in internal memory. If I want to add a book or document from a non Amazon source directly to the Tab I have to save it into that folder in internal memory. The internally memory folder Android/data/com.amazon.kindle/files still shows all the books on the device even though most of them have been moved to the ext SD card according to the Storage detail as noted above.