Two collections of Diana Wynne Jones’ books are on sale for nice prices in the UK: the three books in the The Land of Ingary Trilogy, which includes Howl’s Moving Castle, for £1.99; and all seven Chrestomanci books, on sale for £4.99. Sadly, these aren’t on sale in the US, at least as of now.
Oh, and if you just need one or two Chrestomanci titles, they are also on sale individually right now for only £2.99 each. Links to the series webpages (may have a couple of strays)
Kindle UK: https://www.amazon.co.uk/s?k=diana+wynne+jones+chrestomanci&i=digital-text&crid=1842VZPPTEZ8D
Kobo UK: https://www.kobo.com/gb/en/series/the-chrestomanci-series