Knowledge media face an awkward chasm between theories.
The old theory was naive transmissionism: "I'll convey this knowledge by telling you about it." That's effectively books' learning model.
But we know that model's wrong: learning is an active process of assimilation.
Conversation
Books (and videos and lectures) sometimes work anyway, but because the learner's doing the heavy lifting—making connections, posing & answering questions, etc
In apprenticeships and great classrooms, the new theory (constructivism) operates: teachers foster active assimilation.
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But what's the equivalent of a "book" which was composed using an effective theory of how its reader will learn? We don't know.
It's a rock and a hard place: we know the old theory's wrong; we don't know how to make media which operate under our new theories.
Exciting times.
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They're sometimes effective, insofar as they support readers in thinking through the ideas they're learning. But they're a blunt tool. Barriers to thinking-it-through include being: boring, shallow, too complex, weak on metacog support. Note all these are per-student relative!