While How To Play Good Opening Moves
is certainly a book for beginners and early intermediates, I felt that it wasn't rigorous enough to take command of the opening. When I was younger, the Queen's Gambit was my weapon of choice (nobody in my club was hypermodern enough to play the King's Indian, so they all countered with 1. ... d5), but I never knew how to equalize if Black accepted the gambit. Unfortunately, Mednis only devotes a paragraph to winning the pawn or fortifying one's position if Black accepts, so I never really became comfortable in that position. I suppose I shall have to dig up a book on the Queen's Gambit in order to bring me up to speed on modern d4 theory.