Three JK Rowling “Hogwarts Library” books, which are tie-ins to the Harry Potter series, and are usually mentioned (or even essential elements) in the main seven books, have dropped in price right now, both in the US and the UK.
Quidditch Through the Ages
Kindle US, $1.99: https://www.amazon.com/dp/B01F3ET2QU
Kindle UK, £0.99: https://www.amazon.co.uk/dp/B01F3ET2SS
also at Kobo US, $1.99: https://www.kobo.com/us/en/ebook/quidditch-through-the-ages-4