For a beginner (like me, who merely dabbled in RW flying), I found the two pertinent books by Helen Krasner ("Flying Helicopters" and "Helicopter Pilot's Companion") quite helpful and very hands-on (but IMO not just focused on RW principles of flight). "Cyclic and Collective" by Shawn Coyle is an excellent read, too, but more advanced as already mentioned (can't tell about the 250 hrs threshold, though, as I maybe have a tenth of that). "FAA Helicopter Flying Handbook" is an informative source as well. Another recommendation (but admittedly not on principles of flight) would be "Fatal Traps for Helicopter Pilots" by Greg Whyte.