Why is “der” used in this excerpt instead of “er”?

Question

From the Unendliche Geschichte:

Schließlich klappte der sein Buch wieder zu – wie vorher, mit dem Finger zwischen den Seiten – und knurrte:

Was der supposed to be er or is it a new construction I haven’t seen yet?

Answer 1

A little more context:

1. Der Junge wußte nicht recht, was er tun sollte, deshalb blieb er einfach stehen und schaute den Mann mit großen Augen an.
2. Schließlich klappte der sein Buch wieder zu - wie vorher, mit dem Finger zwischen den Seiten - und knurrte:

There are two male characters. Using the pronoun "er" is therefore ambiguous, since it could refer to either the boy or the man, with preference to the subject of the preceding sentence, in this case the boy. The pronoun "der" is used in such cases to refer to the object of the preceding sentence, in this case the man. This distinction is necessary when both the subject and the object have the same genus. It can also occur with mixed genera, again referring to the object of the previous sentence.

A side effect of this construction is that in later sentences, a simple "er" will refer to this character, resulting in a change of subject.

1. Der Junge (= main subject) wußte nicht recht, was er tun sollte, deshalb blieb er (= main subject, der Junge) einfach stehen und schaute den Mann mit großen Augen an.
2. Schließlich klappte der (= der Mann; main subject becomes der Mann) sein Buch wieder zu - wie vorher, mit dem Finger zwischen den Seiten.
3. Er (= main subject der Mann) sagte kein Wort.

Well, I'm still a little unsure whether the focus really changed, or if it only became more ambiguous. At least the focus moves a little bit. This uncertainty may be the reason why this construction is not used often. The referred character should be made clearer by giving additional hints to the reader.

Answer 2

Der isn't used as an article but as a demonstrative pronoun here. The difference to the actual demonstrative pronoun dieser is subtle.

The following sentences are unambiguous:

Zwei Männer saßen gegenüber, einer las. Schließlich klappte dieser sein Buch wieder zu.

Two men sat vis-à-vis, one (of them) read. Eventually that one closed his book.

Zwei Männer saßen gegenüber, einer las. Schließlich klappte der sein Buch wieder zu.

Two men sat vis-à-vis, one (of them) read. Eventually he (the one reading) closed his book.

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While the following sentence is stilistically inferior, as we talked about two men before:

Zwei Männer saßen gegenüber, einer las. Schließlich klappte er sein Buch wieder zu.

(ist der andere plötzlich verschwunden?)

Two men sat vis-à-vis, one (of them) read. Eventually he closed his book.

(did the other man magically disappear?)