They're not a series, they're just two books published by the same author. Both are good, but they're good for different reasons. 7DCS focuses on common practical mistakes that players make and ways to remedy those. Chess for Zebras is more focused on various methods of thinking about chess... but it's actually pretty difficult to categorize/summarize what that book is about. They're both worth your time, and they don't need to be read in any order. You can read one without having to read the other.
Chess For Zebras & 7 Deadly Chess Sins
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Both excellent books. Very different from most others. Very enjoyable reads for me.
For people who have read this books, are they a series? was thinking of reading chess for Zebras first