Edward Eager! Oh, wow, I loved his books. Half Magic was, I think, my favorite. I also remember really liking the one when the same kids from Half Magic (who are the parents of Roger Ann Eliza Jack? maybe.) go to the lake for the summer and find the magical turtle. I can't remember the title, but it might be Magic by the Lake.

I'm going to have to raid my sister's bookshelves and reread all these now.