2. Magic By The Lake
3. Knight's Castle
4. The Time Garden
5. Magic or Not?
6. The Well-Wishers
7. Seven-Day Magic
I'd heard so many people mention the first book that I decided I wanted to read it. I found a nice boxed set of all seven on eBay for $10 and bought it. Note that I have listed the books in internal chronological order rather than publishing order which swaps 2 & 3.
All of these books are about children (usually a batch of four or five) finding something magic and using it for adventures. Books 1 & 2 go together, then 3 & 4, then 5 & 6, and then 7. It's clear that there was meant to be a sequel like the others, but he died two years after 7 was published (at age 53) and I guess he just didn't get to it. They're pleasant reading, not challenging.
I noticed a lot more word play and allusions in the first four books. The last three not only are more literal, but church steps in. They don't do magic on Sunday and in books 5 & 6, a church is peripherally involved in the story. I don't know if he changed his beliefs between 4 & 5 or if the times had changed. (I'm not finding much about him online.)
The boxed set I have was from Scholatic in 1999 and has tacky cartoony illos on the fronts & backs of the books (plus the box) but the original very good illos are inside.
I'd say this was still a good set of books to read to kids or for kids to read themselves. They need to be able to deal with the past.