Hvora bók vilt þú kaupa?

Because hvora has to agree with bók in gender, number, and case.

• bók is feminine
• it is singular
• here it is the direct object of kaupa, so it is in the accusative

The feminine singular accusative form of hvor is hvora.

Hvor usually means which of the two, while hvaða is a more general which.

So:

• Hvora bók vilt þú kaupa? = most naturally Which of the two books do you want to buy?
• Hvaða bók vilt þú kaupa? = Which book do you want to buy? in a more general sense

Even if the English translation just says which book, Icelandic may choose hvora when the context is a choice between two things.

Because the singular form of bók is the same in both the nominative and the accusative.

So even though the noun is in the accusative here, it still looks like bók. The case is easier to see on hvora, which changes form.

Vilt is the present tense, second-person singular form of vilja, which means to want.

So:

• ég vil = I want
• þú vilt = you want

It matches the subject þú.

Because Icelandic main-clause questions usually follow verb-second word order.

In this sentence:

• Hvora bók = question phrase
• vilt = finite verb
• þú = subject
• kaupa = infinitive

So after the question phrase, the finite verb comes before the subject.

Because kaupa is an infinitive that depends on vilt.

Vilja works like want to in English:

• vilt = want
• kaupa = to buy

In Icelandic, when you have a finite verb like vilt plus an infinitive like kaupa, the infinitive often comes later in the clause.

Yes. Viltu is a very common contracted form of vilt þú.

So these mean the same thing:

• Hvora bók vilt þú kaupa?
• Hvora bók viltu kaupa?

The contracted form often sounds more natural in everyday speech.

Usually, yes.

Using hvor/hvora normally suggests a choice between two items. If you are choosing from a larger group, Icelandic more often uses hvaða instead.

So the choice of hvora gives you a small grammatical clue about the situation.

Not as a normal direct question.

For a main question, Icelandic normally wants the finite verb in second position, so:

• Hvora bók vilt þú kaupa? = normal

A sequence like hvora bók þú vilt kaupa sounds more like part of a longer sentence or a subordinate clause, not a standard independent question.

In modern Icelandic, hv is usually pronounced much like kv, so hvora begins roughly like kvora.

The letter þ is pronounced like the th in thing, so þú begins with that voiceless th sound.