Why are opening books so popular? As title above, why are opening books so popular? I am sure that most players know that middlegame and endgame books are ultimately much more important for improving one's chess strength. That is not to say that openings are not important, but it seems to me that a good 80% or so of chess literature are opening / repertoire books, and that means that these are the most bought ones.
I think there are number of reasons why these are so popular:


It is easy to start learning openings. You can read about ideas and memorize some lines. It is not so easy to learn middle game or end game in similar fashion.
A lot of players want to get an edge from the start- hence the focus on openings.
People enjoy reading them
They are easier to write than good middle game books (as indicated by different chess authors)


It is questionable if middlegame or endgame books are more important. There is an argument to be made that you need something other than books to learn good middlegame / endgame skills. Openings on the other hand can be easily learned from books.