Published order (or written order, in the event those two conflict, as in this case) is
always the way to read a series (IMO) unless the author(s) expressly indicates otherwise. Internal series chronology is of relatively little importance. Otherwise, the term
prequel would have no meaning.
Think about what order someone would have read them in had they started reading them
before they were all published.
So, since this is the 2nd book
written in the series, there's a valid concern that some important background may be missing from the first written book (I just happen to think it's a very small concern since it was van Gulik's
first). But I think it would be silly to expect someone to verify that a book is the very first according to the series' own internal chronology before nominating it for the book club.