Magic by the Lake is the first sequel to Half Magic, and I only wish I had known about it as a child.
The Story: The same four kids as in the first book are back for another magical adventure. They are off to spend several weeks at a lovely little lake house. Little do they know that the lake will bring them more magical adventures, courtesy of a very grumpy magical turtle (because, if you didn't know, all turtles have some magic to them).
They have some more wonderful magical adventures, though they learn to abide by some simple rules set forth by the turtle. Most of their wishes have to be suitably watery in nature, but they don't find that to be too limiting.
The Good: These kids are so much fun. They are just naughty enough to be fun, without being reckless (most of the time), and again, the writing is so much fun. I could read this multiple times happily.
The Bad(ish): Just more of the same, since it was written so long ago, the kids aren't always doing things that we wold deem appropriate. That is all.
The Verdict: I don't think you can go wrong with any books from this series. We started reading this book again, but then Evalina got distracted by some other books (I did, too) so we didn't finish the second reading. Yet. I'm sure we'll come back to it.