This review will be short. I apologize for that. I'd go into why, but this isn't my personal blog.
I love Edward Eager's books, particularly Half Magic, which Magic by the Lake is a sequel to. Eager's style is purposefully similar to E. Nesbit, which he greatly admired, and his stories are charming, clever, inventive, and funny, with realistic children and magic that has unexpected consequences.
Unfortunately, Magic by the Lake is not nearly his best book. It's a bit too episodic, without much of an overarching plot to tie the magical events together. The children from Half Magic travel with their mother and stepfather to a lake for the summer, and almost immediately they discover a talking turtle who explains to them the rules for the lake's magic. Thereafter the children have adventures that include mermaids, pirates with buried treasure, and so forth.
It's a quick read, but unless you're just wanting to read everything Edward Eager ever wrote, you're much better off reading Half Magic without needing to read this one too.