Learn German: Ich lese das Buch oft. - I read the book often.

Yes! In this sentence, das Buch is the direct object—the thing that is being read. That means it must be in the accusative case. Because Buch is a neuter noun, its definite article das stays exactly the same in the accusative case as it is in the nominative (subject) case.

What would happen if I were reading a masculine noun, like "the text" (der Text)?

That is where the accusative case becomes visible! Masculine nouns change their article from der to den when they are the direct object. So the sentence would become: Ich lese den Text oft. Since das Buch is neuter, its article gets to stay as das.

Why is oft at the very end of the sentence?

In English, we usually put "often" before the verb ("I often read the book"). In German, short adverbs of frequency like oft generally come after the direct object if the object is specific (like das Buch). So German prefers the order: Subject (ich) + Verb (lese) + Object (das Buch) + Adverb (oft).

Is the verb lesen (to read) completely regular?