Not sure as I've not read them all, but this might be the best of the series. Judging by the note inc. in the acknowledgements, it might be the author's favorite, too.
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I do like this series. If I was a kid looking for something more substantial than Goosebumps, I would love it. The adventure and humor are enough, but then's there are themes. Basically, it's Jekyll & Hyde for kids, regarding our inner & outer nature, or our good & evil impulses, but then there are also themes about bullies, families, friends, secrets, courage....
(Copied to each.)