Annoyingly, Amazon have a habit of reverting books from 'free' to 'not free'! If you find this to be the case, you can email them asking them to price-match it to free, as it is free on other platforms. Or just get it for free via Kobo, iBooks, etc...
Enjoyed the book? Feel overwhelmed by guilt that you read it for free? Don't worry - you can salve your conscience by buying an NFT to treasure forever (or sell on at inflated price when I am massively famous, obviously).