Inače kupujem knjige.

Yes. Inače is a very useful word with a few related meanings depending on context. It can mean:

• usually / generally
• otherwise
• sometimes something like as for that / apart from that

In this sentence, the meaning is clearly the usually / generally one.

Because Serbian often omits subject pronouns when they are clear from the verb form.
The ending in kupujem already tells you the subject is I.

So:

• kupujem = I buy / I am buying
• ja kupujem = I buy, but with extra emphasis on I

In neutral sentences, leaving out ja is completely normal.

Kupujem is the 1st person singular present tense form of the imperfective verb kupovati.

So:

• kupovati = to buy in an ongoing, repeated, or habitual sense
• kupujem = I buy, I am buying, or I usually buy, depending on context

Because of inače, the sentence is understood as a habit: I usually buy books.

This is about aspect, which is very important in Serbian.

• kupovati / kupujem = imperfective → repeated, ongoing, habitual action
• kupiti / kupim = perfective → a completed action, one whole purchase

Since the sentence talks about what you generally do, Serbian uses the imperfective form: kupujem.

So Inače kupujem knjige is natural for a habit, while kupim would not fit that meaning.

Because knjige is the direct object of the verb kupujem, and direct objects normally go in the accusative case.

The singular noun is:

• knjiga = book

Its plural is:

• knjige = books

In this case, the accusative plural has the same form as the nominative plural, so you see knjige.

It is talking about a general habit.

Serbian present tense can describe:

• something happening now
• something that happens regularly
• a general fact

The word inače pushes the meaning strongly toward habit / usual behavior, so the sentence means something like I usually buy books, not I am buying books right now.

No, Serbian word order is fairly flexible, but some orders sound more neutral than others.

Inače kupujem knjige is a very natural, neutral order.

You could change the order for emphasis, for example:

• Knjige inače kupujem. → emphasis on books
• Ja inače kupujem knjige. → emphasis on I

But for a learner, Inače kupujem knjige is the safest and most natural version.

Yes. Obično kupujem knjige is also correct and often even more directly equivalent to I usually buy books.

A rough difference is:

• obično = more straightforwardly usually
• inače = generally / normally / as a rule, sometimes with a slightly broader nuance

In many everyday situations, they can be very close in meaning.

Yes, you could say knjigu, but it would change the meaning.

• kupujem knjige = I buy books in general
• kupujem knjigu = I buy a book / I am buying a book

So the plural knjige makes the statement more general and fits well with the habitual meaning created by inače.