How to open multiple Kindle books at the same time?
I haven't tested this specifically with Kindle, but it works for most apps that don't normally open a second instance...

In Terminal
open -n
(note the space after -n) 
then drop the Kindle app onto the Terminal window to auto-fill the path to it &amp; hit  Return⏎  

back in Terminal,  Opt ⌥   ↑  then  Return⏎  will repeat that command &amp; should open a second instance.