I like to break it up into 5 general categories:
openings, strategy, tactics, endgame, annotated game collection.
Strategy is pretty broad, from ideas like pawn structure to positional sacrifices to setting up an attack.
Pick your weakest area, or something you're interested in. Avoid picking what you've already spent the most time learning or practicing.
Oh, and for example an annotated collection for your rating would be a book like this:
https://www.amazon.com/Understanding-Chess-Move-John-Nunn/dp/1901983412
What should I study other than tactics and endgames?? (I have a 1394 USCF rating.)