4 volumes - Winner of the Italian Chess Federation’s Golden Award.
These books are both primers and opening references that provide essential knowledge on all chess openings. The authors explain the basic plans and ideas for both sides in a way that will help develop a solid understanding of fundamental opening ideas. They explain the similarities with other openings as well as the differences and detail various middlegame plans that apply after the opening has ended.
Vol. 1 covers 1.e4
Vol. 2 covers 1.d4 d5 / 1.d4 various / Queen Gambits
Vol. 3 covers the Indian Defenses
Vol. 4 covers 1.c4 / 1.Nf3 / Other First Moves
I would recommend that, after selecting what openings you want to play, you thoroughly review the material in the books then collect a database of master games with those openings. Play over them with a careful eye to how the games flow out of the openings and the types of endings that may develop. After you analyze 25 or so master games with your chosen opening you should have begun to develop a feel for how it should be played.