Grandmaster Richard Palliser studies the Sicilian Najdorf in a simplistic way, introducing the crucial initial moves and ideas and taking care to explain the reasoning behind them, something that has sometimes been neglected or taken for granted.
Good introduction to the main lines after 5 . . . a6 in the Sicilian. Blizzard of theory and too many options, but this is a decent road map on what to play and what to expect in return.