As a 1600 player, I’d say two for the openings. A good ideas book, back in the day Reuben Fine’s “Idea’s Behind the Chess Opening” and an MCO type enclopedia. The first allows one to find what openings feel right, the second allows deeper exploration.

I never read much of the middlegame books, so can’t comment. I tended to use game collections for this, e.g., Alekhine’s and Fischer’s books.

For the endgame, similar to the openings, a short introduction and a more detailed account, in my case, 30 years ago Keres’ and Fine, respectively.

Nowadays a decent chess playing program is useful also. I use ChessGenius ’cause it runs on my Win98 system.

My current opening books are MCO-14 for detail and “Understanding the Chess Openings” by Sam Collins. The latter is not as good as the Fine , but is up to date.

For the endgame, I have only a general introduction, Y. Seirawan’s “Winning Chess Endings”, which I will begin to study this weekend, so I can’t comment on its merits.