Really the best stuff is the older books, like The Making Of Star Trek Deep Space Nine by Judith and Garfield Reeves-Stevens and the DS9 Companion, if only because the book was written and published during the show, or very shortly after, and the authors had access to the production, and could see what was going on on an everyday basis. It wasn't like now where, even if they interviewed people, the information would be based on old memories, or the older books in order to get a sense of what went on during the series.
Now then back around 2000/01, there was an ebook released on the Making of Trials and Tribble-ations that has never been reprinted in paperback or Hardcover.