Moving Water is the most comprehensive book on how current affects fly fishing, the good and bad of drag, which casts and techniques to use when, and much more. 
The book covers a variety of topics, including:

• Nymph fishing and accounting for variations in current speed at different depths
• How even small movements of water can change your fly's movement, and what to do about it
• The effects of turbulence on water temperature and oxygen content
• How mayflies, stoneflies, and midges are affected by the water in which they live
• Trout's evolutionary and day-to-day responses to current
• Fishing drop-offs, midstream obstructions, and sweepers