When I make a book free, I would make it free on all platforms, but Amazon does not allow this for MOST authors. Amazon will, however, match to free, so if that is your platform of choice, click the Amazon link even if it says 99c below because it may be free by now.
Some books/price changes may be in the process of updating or going live. Affiliate links are used for Amazon
First Installments (i.e. here there be cliffhangers)