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 & hit Return⏎

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