User`s guide

Publishing on Kindle: Guidelines for Publishers
In portrait mode, the two pages will be rendered separately as shown below:
If none of the properties are specified, Kindle assumes facing-page-left and facing-page-right
for alternate pages based on the book’s writing mode.
The following example assumes primary-writing-mode equals horizontal-lr or vertical-lr.
Example:
<spine>
Kindle Publishing Guidelines Amazon.com 50