What book by Jones would you recommend starting with?

It's been ages.. Howl's Moving Castle is the most popular, and had an anime by Studio Ghibli

Normally you'd start with the first book from the Chrestomanci series, but I think The Magicians of Caprona (1980) and Witch Week (1982) might be the best for a new fan.

It's shame she passed away before writing much more and getting the fame she deserved :(

She had a unique and cozy style and way of world building and character design, where there is challenge and conflict but not deep malice or misery, if that makes sense..