الكتاب مثير.

• الكتاب = the book
• مثير = interesting / exciting / stimulating

Grammatically, this is a nominal sentence in Arabic:

• الكتاب is the subject (المبتدأ)
• مثير is the predicate (الخبر)

So the whole sentence means The book is interesting.

In Arabic, the verb to be is usually not expressed in the present tense.

So:

• الكتاب مثير = The book is interesting

But in the past or future, Arabic does use a verb:

• كان الكتاب مثيرًا = The book was interesting
• سيكون الكتاب مثيرًا = The book will be interesting

This missing is is completely normal in Arabic.

Because الكتاب is the definite subject: the book.

But مثير here is not an adjective attached directly to the noun in the phrase the interesting book. Instead, it is the predicate of the sentence: interesting.

So:

• الكتاب مثير = The book is interesting
• الكتاب المثير = the interesting book

That difference is very important:

• predicate adjective: الكتاب مثير
• attributive adjective: الكتاب المثير

In Arabic, an adjective only matches a noun in definiteness when it is directly modifying that noun.

Compare:

• الكتاب مثير = The book is interesting
  Here مثير is the predicate, so it does not take الـ.

• الكتاب المثير = the interesting book
  Here المثير is an adjective modifying الكتاب, so it does take الـ.

So the lack of الـ on مثير helps show that this is a full sentence, not just a noun phrase.

A careful full pronunciation in Modern Standard Arabic is:

al-kitābu muthīrun

A few useful notes:

• الكتاب is pronounced al-kitāb because ك is a moon letter, so the l in الـ is pronounced.
• مثير is pronounced muthīr

In everyday reading and speaking, the final case endings are often not pronounced, especially at pause, so you may hear:

al-kitāb muthīr

Both are useful to recognize.

In fully vocalized MSA, the sentence is:

الكتابُ مثيرٌ

Both words are in the nominative:

• الكتابُ: subject (مبتدأ) → nominative
• مثيرٌ: predicate (خبر) → nominative

Why does مثير have tanwīn in full spelling?

• Because it is indefinite
• So in full vocalization it becomes مثيرٌ

In normal unvoweled writing, these endings are usually omitted:

• الكتاب مثير

It is best understood here as an adjective used as the predicate.

So in:

• الكتاب مثير

مثير means interesting / exciting and describes the subject.

Arabic often uses adjective forms this way in nominal sentences:

• الجو جميل = The weather is beautiful
• الفيلم طويل = The movie is long

So yes, it is an adjective, but in this sentence it functions as the predicate.

The normal, neutral order is:

الكتاب مثير

This is the most common way to say The book is interesting.

Arabic can sometimes change word order for emphasis or style, but for a learner, the standard pattern to remember is:

• definite subject + indefinite predicate

So this sentence is an excellent model:

• الطالب ذكي
• البيت كبير
• الكتاب مثير

You would say:

كتابٌ مثيرٌ = an interesting book

Here مثير is a true adjective modifying كتاب, so both are:

• indefinite
• singular
• masculine
• nominative if standing alone

Compare the three patterns:

• كتابٌ مثيرٌ = an interesting book
• الكتابُ المثيرُ = the interesting book
• الكتابُ مثيرٌ = The book is interesting

This is a very useful contrast to learn early.

Then the adjective/predicate must agree in gender.

For example:

• القصة مثيرة = The story is interesting
• الرواية مثيرة = The novel is interesting

So:

• masculine singular: مثير
• feminine singular: مثيرة

Arabic adjectives agree with the noun in gender and number, whether they are used attributively or as predicates.

Not always. مثير can also mean:

• exciting
• stimulating
• sometimes provocative, depending on context

So الكتاب مثير could mean:

• The book is interesting
• The book is exciting

The exact English translation depends on context, but interesting is often the safest basic meaning for learners.