I read The Westing Game for the first time for a fic gift exchange a few years ago. I enjoyed it very much but ended up writing for another book. It's very clever.

My choice is The Melendy Quartet, by Elizabeth Enright, which was published beginning in the early 1940s. I read them in elementary school, and then again to write a Yuletide story back in the day. A fun series, perfect for ages 8-12, and the young at heart of all ages. I remember I also loved her Thimble Summer and Gone-Away Lake.