هذا كتاب مناسب.

Because هذا means this, and here it is introducing the whole statement: This is a suitable book.

In this sentence, هذا is not directly attached to كتاب as part of a single noun phrase like this book. Instead, the sentence is structured more like:

• هذا = this
• كتاب مناسب = a suitable book

So the sentence is literally closer to This is a suitable book.

That would mean something different.

• هذا كتاب مناسب = This is a suitable book
• هذا الكتاب المناسب = This suitable book or this appropriate book

In your sentence, كتاب is indefinite: a book, not the book. That is why it does not have الـ.

So this is a full sentence, not just a noun phrase.

No. In the present tense, Arabic usually does not use a word for is in sentences like this.

So:

• هذا كتاب مناسب literally looks like this suitable book
• but it means This is a suitable book

This is called a nominal sentence in Arabic.

If you wanted past or future, Arabic would use other forms, but in the present tense the is/am/are is normally omitted.

مناسب means suitable, appropriate, or fitting.

So كتاب مناسب means:

• a suitable book
• an appropriate book

The exact English translation depends on context, but suitable is a very natural choice here.

Because adjectives normally come after the noun in Arabic.

So:

• كتاب مناسب = literally book suitable
• natural English = suitable book

This is the normal Arabic pattern:

• noun first
• adjective second

Because the adjective must match the noun in definiteness.

Here:

• كتاب = indefinite, a book
• مناسب also stays indefinite, because it describes كتاب

So Arabic matches them:

• كتاب مناسب = a suitable book
• الكتاب المناسب = the suitable book

This matching is a very important rule in Arabic adjectives.

Because كتاب is a masculine singular noun, and هذا is the masculine singular form of this.

• هذا = this for masculine singular
• هذه = this for feminine singular

Since كتاب is masculine, هذا is the correct choice.

With full case endings, it is:

هَذَا كِتَابٌ مُنَاسِبٌ

Transliteration:

hādhā kitābun munāsibun

Why the -un endings?

Because كتاب and مناسب are in the nominative here and indefinite, so they take tanwīn ḍamm.

In normal everyday reading and speaking, these endings are often not pronounced, especially in pause, so you may hear:

hādhā kitāb munāsib

It functions as the predicate of the sentence.

A simple way to analyze it is:

• هذا = subject-like element, this
• كتاب مناسب = predicate, a suitable book

So the sentence means that this is being identified as a suitable book.

More traditionally in Arabic grammar, هذا is the مبتدأ and كتاب مناسب is the خبر.

Yes in meaning, yes in grammar.

The difference is only that:

• هذا كتاب مناسب is written without short vowels, which is how Arabic is usually written
• هذا كتابٌ مناسبٌ shows the full case endings and pronunciation

So they are the same sentence; one is just more fully marked.

Yes. مناسب is a very common adjective and can describe many things:

• وقت مناسب = a suitable time
• مكان مناسب = a suitable place
• حل مناسب = a suitable solution

It changes form to match gender and number when needed, but the basic meaning stays suitable/appropriate.

A natural MSA version is:

هذا ليس كتابًا مناسبًا

Here ليس is used for is not.

Notice that after ليس, the predicate part takes accusative endings in full vocalization:

• كتابًا
• مناسبًا

Without case endings, it is usually just written:

هذا ليس كتابا مناسبا or هذا ليس كتاب مناسب depending on the writing style, though the fully correct vocalized form is كتابًا مناسبًا.