If book one is only OK, then I'm not likely to bother with book 2 and on as there's plenty else to read, especially if I'm being asked to pay money for it. I might consider it if it's free (or part of a bundle with sufficient other books of interest to me that it's effectively free).
I might reconsider if the book is a first book by a new author on the grounds the author may not be writing well yet. If it's an established author, then almost certainly not.
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