Even with respect to computer science though, I think the point is dead wrong. For example, take an awesome introductory textbook like Sipser's Introduction to the Theory of Computation. You can work through that book doing all the exercises in a few weekends, and it is such a pleasure to read that it's hard to put it down. There's no way you could learn as much as efficiently by randomly jumping from topic to topic on Wikipedia and reading online resources.

A good textbook is still pretty much the greatest bargain in the known universe.