I discovered Diana Wynne Jones when I was 8 or 9, with Charmed Life, and I think I read about 8 of her books, mostly ones published in the 1970s and ones published by Puffin Books. Charmed Life and The Ogre Downstairs were my favourites and I think I read them more than once. I acquired ex library and secondhand copies of some of her later books, and I think I read Howl's Moving Castle and Hexwood in my early 20s. I didn't discover that there were more Chrestomanci books until my late 20s.
In my 30s and 40s I've reread and read for the first time quite a few of her books, but still have quite a few to reread and some to read for the first time.
It took me ages to get round to Dark Lord of Derkholm which I think is a lovely portrait of family life with teenagers, and Year of the Griffin is a really fun campus novel. I like the ones which mix quite ordinary details of family life in with all the magic.
DS1 has read all the Chrestomanci books - I still have to get to the last two to be published. He's read Howl's Moving Castle and the two sequels, plus The Ogre Downstairs and Archer's Goon. I need to dig out my copy of Dogsbody for him, or find him another copy if I see a secondhand one (as he really likes stories with canine characters and I think this is a lovely one).
On J K Rowling, I think DWJ is a much better writer and I'm sure she was inspired by DWJ and Jill Murphy's The Worst Witch (she's a few years older than me and may not have read all the Worst Witch sequels as a kid - I have most of them but still haven't got there). As well as possibly by other boarding school stories.