أنت لا تريد الكتاب؟ بلى.

Because بلى is the special word used to contradict a negative question or negative statement.

In this sentence, the question is negative: أنت لا تريد الكتاب؟
So بلى means:

• Yes, on the contrary
• Actually, I do

That is why it is the correct reply if the speaker does want the book.

Compare:

• أنت لا تريد الكتاب؟ بلى. = Don’t you want the book? — Yes, I do.
• أنت لا تريد الكتاب؟ نعم. = Don’t you want the book? — Yes, I don’t.
  In careful MSA, نعم confirms the negative.

So this is one of the most important uses of بلى in Arabic.

Yes. Arabic verbs already show the subject, so أنت is often optional.

• تريد can mean you want (masculine singular)
• so أنت لا تريد الكتاب؟ and لا تريد الكتاب؟ can both work

However, أنت may be added for:

• emphasis
• clarity
• contrast
• making it very explicit that the speaker means you

It is especially helpful here because the form تريد can also be read as she wants in another context. Adding أنت removes that ambiguity.

Because Arabic does not use do-support the way English does.

In English, we say:

• You do not want the book
• Do you not want the book?

But in Arabic, negation is done directly with لا before the present-tense verb:

• لا تريد = you do not want

So the structure is more literally:

• أنت لا تريد الكتاب = You not-want the book

That is completely normal Arabic grammar.

In Arabic, a yes/no question can often be shown simply by:

• intonation in speech
• a question mark in writing
• context

So أنت لا تريد الكتاب؟ can mean Don’t you want the book?

Arabic also has question particles such as هل and the interrogative hamza أَ. A more explicitly marked version could be:

• ألا تريد الكتاب؟ = Don’t you want the book?

So the sentence you have is natural, but it relies on question intonation/punctuation instead of an added question word.

الكتاب has الـ, so it means the book, not a book.

• الكتاب = the book
• كتابًا = a book as a direct object

So:

• أنت لا تريد الكتاب؟ = Don’t you want the book?
• أنت لا تريد كتابًا؟ = Don’t you want a book?

The first refers to a specific book; the second is more general.

It is the direct object of تريد, so it is in the accusative case.

If full case endings are written, it would be:

• الكتابَ

So a fully vocalized version is:

• أَنتَ لا تُرِيدُ الكِتابَ؟ بَلَى.

In everyday Arabic writing, these final short vowels are usually omitted, so you normally just see:

• أنت لا تريد الكتاب؟ بلى.

The final ى here is alif maqṣūra, a special way of writing a final long ā sound in some words.

So بلى is pronounced balā, even though it ends in ى rather than ا.

This is just the standard spelling of the word. Learners usually just have to memorize it:

• بلى = pronounced balā

Then the verb would change to the feminine singular form:

• أنتِ لا تريدين الكتاب؟ بلى.

Fully vocalized:

• أَنتِ لا تُرِيدِينَ الكِتابَ؟ بَلَى.

So the difference is:

• to a man: أنتَ لا تريدُ الكتابَ؟
• to a woman: أنتِ لا تريدينَ الكتابَ؟

The reply بلى stays the same.

A word-by-word breakdown is:

• أنت = you
• لا = not
• تريد = want
• الكتاب = the book
• بلى = yes, on the contrary

So the structure is roughly:

• You not want the book? — Yes, on the contrary.

That literal breakdown helps show how Arabic builds the sentence, even though the natural English translation is smoother.