bookworm,
I had a suspicion that the authors (Anderson and Eberhardt "Understanding Flight') were trying to throw something out because it wasn't accurate to the nth degree.
After all, the planemakers themselves use it OK.
Enough of them have written books based on their time in industry showing it's good enough for them.
eg Richard Shevell (Douglas) "Fundamentals of Flight"
Ed Obert (Fokker) "Aerodynamic design of Transport Aircraft"