Learn Hardcore Japanese: kono syousetu ha totemo omosiroi desu.

この小説: this novel (near me, the speaker)
その小説: that novel (near you, the listener, or something just mentioned)
あの小説: that novel over there (far from both speaker and listener, or a third thing)

All three must be followed by a noun:

この小説 – this novel
その本 – that book
あの映画 – that movie (over there / that one we both know about)

You cannot use この / その / あの without a noun (unlike English this/that sometimes). For that, you use これ / それ / あれ instead.

Why is 小説 used instead of 本? Can I say この本はとても面白いです?

小説 (しょうせつ) means novel specifically: a work of fiction in prose.

本 (ほん) means book in general: could be a novel, textbook, manga, picture book, etc.

So:

この小説はとても面白いです。
This novel is very interesting. (You are clearly talking about a novel.)
この本はとても面白いです。
This book is very interesting. (More general. It might be a novel, but the word itself does not specify.)

Both sentences are grammatically correct; the choice depends on how specific you want to be.

What is the role of は in this sentence?

は is the topic particle. It marks what the sentence is about.

In この小説はとても面白いです, the topic is この小説 (this novel). A very literal way of feeling the structure is:

As for this novel, it is very interesting.

Important points about は:

It does not necessarily mark the grammatical subject; it marks the topic.
It sets up a frame; everything that follows is information about that topic.

Also, in writing it is は, but when it’s used as the topic particle, it is pronounced wa, not ha.

Why is は pronounced like wa here and not ha?

The character は has two roles:

As a normal syllable in words (e.g. はな for nose/flower), it is pronounced ha.
As the topic particle, it is written は but pronounced wa.

This is just a historical spelling rule that modern Japanese keeps:

私は学生です。 → pronounced watashi wa gakusei desu
はな (nose/flower) → pronounced hana

So in この小説はとても面白いです, は is the topic particle, so it is pronounced wa.

What does とても mean, and can I use other words instead, like すごく?

とても is an adverb that means very. It makes the adjective stronger:

面白い – interesting
とても面白い – very interesting

Other common words with a similar function:

すごく面白い – really / super interesting (more casual)
めっちゃ面白い – super / insanely interesting (very casual, slangy)
かなり面白い – quite interesting

Nuance and politeness:

とても is neutral and polite enough for most situations.
すごく is also common, but more casual in feel.
めっちゃ is clearly casual and regionally associated with Kansai, though widely used.
たいへん面白い sounds more formal or written.

In this sentence, とても matches the polite です style nicely.

Does とても need a particle before it? Why is there nothing between 小説は and とても面白い?

No particle is needed before とても here, because とても is an adverb.

Adverbs in Japanese, like in English, usually attach directly to adjectives or verbs without particles:

とても面白い – very interesting
すごくきれい – really beautiful
ゆっくり話す – speak slowly

So the structure is:

この小説は (as for this novel)
とても面白い (is very interesting)
です (polite sentence ending)

No extra particle is required between は and とても.

What exactly does 面白い mean? Is it interesting, funny, or both?

面白い (おもしろい) has a wider range than English interesting:

interesting
fun, enjoyable
amusing, sometimes even funny

Context decides the nuance:

For books, movies, games: usually interesting / entertaining / good.
この小説は面白い – This novel is really good / interesting to read.
For things that make you laugh: can be funny.
あの人は面白い – That person is funny / amusing.

So in your sentence, for a novel, very interesting or really good is usually the natural translation, but remember it can also imply that it was enjoyable and entertaining, not just intellectually interesting.

Why is there です at the end? What is its function?

です is the polite copula, used to make the sentence polite and complete in formal or neutral-polite speech.

In sentences with adjectives:

面白い。 – It is interesting. (plain, casual)
面白いです。 – It is interesting. (polite)

Your sentence:

この小説はとても面白いです。
is polite, suitable for talking to someone you are not intimate with: teacher, colleague, acquaintance, etc.

Without です:

この小説はとても面白い。
is grammatically fine but plain; it sounds more casual or diary-like.

Can I use だ instead of です, like 面白いだ?

With i-adjectives like 面白い, you do not attach だ directly.

Correct patterns:

面白い。 – plain, casual
面白いです。 – polite

だ attaches to nouns and na-adjectives, not to i-adjectives:

彼は学生だ。 – He is a student. (学生 is a noun)
この本は静かだ。 (unnatural; 静か is a na-adjective but you would usually say 静かな本)

For your sentence, the natural options are:

この小説はとても面白い。 (plain)
この小説はとても面白いです。 (polite)

面白いだ is incorrect.

Why is there no verb like ある or いる meaning to be? Where is the verb in this sentence?

In Japanese, i-adjectives (like 面白い) already contain the meaning to be X. They can function as predicates by themselves.

So:

面白い can mean is interesting (depending on context).

The structure is:

この小説は – as for this novel
とても面白い – is very interesting
です – adds politeness

There is no separate verb like English to be or French être; the adjective itself acts as the predicate. です is not exactly a verb of existence here; it is mainly a polite sentence ending.

Can I change the word order, like この小説は面白いとてもです?

You cannot freely move とても and です around in that way. Natural options:

As given:
- この小説はとても面白いです。
You can move とても closer to the adjective (already is) or put it before the whole 面白いです, but that is effectively the same place:
- この小説はとても面白いです。 (standard)
- この小説は面白いです。とても。 (possible in spoken Japanese for emphasis, but sounds like afterthought)

You can also move とても before the noun phrase to emphasize, but it sounds a bit marked:

とても面白い小説です。 – It is a very interesting novel.
(Now the structure is: [very interesting novel] + です.)

But 面白いとてもです is ungrammatical. Adverbs like とても normally go right before the adjective or verb they modify.

What level of politeness is この小説はとても面白いです。? How would I say it more casually to a friend?

This sentence is in polite form (です/ます style). It is appropriate for:

talking to teachers
coworkers you are not close to
people older than you
most neutral situations with strangers

To say it more casually, you can:

Drop です:
- この小説はとても面白い。
  Still fairly neutral, but plain/casual.
Also drop the topic particle は (very common in spoken Japanese):
- この小説、とても面白い。
Use more casual intensifiers:
- この小説、すごく面白い。
- この小説、めっちゃ面白い。 (very casual / slangy)

These versions are good with friends or people you talk to in plain form.

Is it better to write おもしろい in hiragana or 面白い in kanji? Which is more natural?

Both are correct:

面白い – kanji + hiragana
おもしろい – all hiragana

In practice:

In adult novels, newspapers, and formal writing, 面白い (with kanji) is common.
In children’s books, beginner materials, or when the writer wants to be easy to read, おもしろい is often used.
On the internet or in casual messages, people might use either; both feel natural.

As a learner, it is good to recognize both, but using 面白い is perfectly normal and will look more like standard adult Japanese text once you are comfortable with the kanji.