Learn German: Der Autor, welcher die Korrektur braucht, ruft den Verlag an. - The author, who needs the correction, is calling the publisher.

welcher (who/which) is a stylistic alternative to der. It is used mostly in written or formal German to sound more elevated or to avoid repeating words (for example, to avoid saying der, der). In everyday spoken German, you would almost always hear Der Autor, der... instead.

Why does welcher end in -er in this sentence?

Relative pronouns take their gender and number from the noun they refer to, but their case comes from their role inside their own clause. Here, it refers to Der Autor (masculine singular) and is the subject of the verb braucht, so it must be in the nominative case. The masculine nominative form of this pronoun is welcher.

Why is braucht at the end of the middle clause, but ruft is in the second position of the main sentence?

The phrase welcher die Korrektur braucht is a relative clause, which is a type of subordinate clause. This forces its conjugated verb, braucht, to the very end. The main clause is Der Autor ruft den Verlag an. In the main clause, the conjugated verb ruft stays in its normal second position, directly following the complete subject Der Autor.

Why is it den Verlag and not dem Verlag? Doesn't calling someone often take the dative case?