It's often more comlex than simply "aileron goes down and creates more drag, so you get adverse yaw". Adverse yaw is the actual term for why you need rudder to "coordinate" a turn. At normal speeds, in a slow airplane with a thick wing, you'll need a fair amount of rudder. If we're talking Cessnas and other piston singles, that is...
But more complex airplanes have varying relationships between rudder and aileron. The effects of adverse yaw vary with wing thickness, sweep, speed, and angle of attack. In big jet airplanes with Yaw dampers, no rudder is needed. In fast jet airplanes that use stabilators, there is no adverse yaw, until you get to very high angles of attack, and even then, you can get to a place in the flight envelope where the Yaw is proverse...
I would recommend "Stick and Rudder" by William Langewische. A truly great book, written over 80 years ago, that presents the basics of flying in clear, concise, terms. You can get the Kindle version for relatively little.
Additionally, for more complex aerodynamics, I would recommend "Aerodynamics for Naval Aviators". Written in the 60s, it remains a great technical primer. You can get it on Amazon, or simply download it for free from the FAA:
http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf