Plus je lis ce roman, plus je le trouve intéressant.

This is a very common French pattern:

Plus..., plus...

It means the more..., the more... in English.

So:

• Plus je lis ce roman = The more I read this novel
• plus je le trouve intéressant = the more I find it interesting

French uses plus at the start of both parts of the comparison.

Here, plus means more.

That is important because plus can also mean no more / no longer in negative sentences, but that is not what is happening here.

Compare:

• Plus je lis, plus j’apprends. = The more I read, the more I learn.
• Je ne lis plus. = I don’t read anymore.

In your sentence, there is no negation. It is a comparison pattern.

Because in this structure, plus introduces the whole clause.

French does not normally build this idea as:

• Je lis plus, je le trouve plus intéressant for The more I read...

Instead, it uses the fixed pattern:

• Plus + subject + verb, plus + subject + verb

So the sentence has to start like this:

• Plus je lis ce roman, plus je le trouve intéressant.

This is just the standard way to express this idea in French.

Le is a direct object pronoun that refers back to ce roman.

So instead of repeating the noun, French replaces it with le:

• ce roman → le

That gives:

• Je trouve ce roman intéressant
• Je le trouve intéressant

In English, this is like saying:

• I find this novel interesting
• I find it interesting

The pronoun avoids repetition.

Because ce roman is the direct object of trouve.

French direct object pronouns are:

• le / la / les

Indirect object pronouns are:

• lui / leur

Here, trouver quelque chose intéressant takes a direct object:

• Je trouve ce roman intéressant.

So when you replace ce roman, you need the direct object pronoun:

• Je le trouve intéressant.

Not lui.

Because it describes ce roman, which is masculine singular.

• ce roman = masculine singular
• therefore intéressant = masculine singular

If the noun were feminine, it would change:

• Cette histoire, je la trouve intéressante.

If it were plural:

• Ces romans, je les trouve intéressants.

So the adjective agrees with the thing being described.

Yes, but in this kind of sentence, trouver often means to find something + adjective, in the sense of to consider / to think it is.

So:

• Je le trouve intéressant = I find it interesting
• more naturally in explanation: I consider it interesting

This is a very common French pattern:

• Je trouve ce film drôle.
• Elle trouve ce cours utile.
• Nous trouvons cette idée bonne.

Yes, but it is not exactly the same structure.

• Je le trouve intéressant = I find it interesting
• Je pense qu’il est intéressant = I think it is interesting

Both are correct, but trouver + object + adjective is especially natural when giving your impression of something.

In your sentence, plus je le trouve intéressant sounds very idiomatic.

Because once the noun has been introduced, French usually avoids repeating it if a pronoun can replace it.

So French says:

• Plus je lis ce roman, plus je le trouve intéressant.

rather than repeating:

• Plus je lis ce roman, plus je trouve ce roman intéressant.

The repeated version is grammatically possible, but it sounds heavy and unnatural.

The comma is very common and helpful because it separates the two parts of the comparison:

• Plus je lis ce roman, plus je le trouve intéressant.

It makes the sentence easier to read.

In informal writing, people may sometimes leave it out, but using the comma is a good idea.

Yes. The plus..., plus... pattern can be used with different tenses depending on the meaning.

For example:

• Plus je lis, plus je comprends.
  The more I read, the more I understand.

• Plus je lisais, plus je le trouvais intéressant.
  The more I was reading, the more I found it interesting.

• Plus tu étudieras, plus tu progresseras.
  The more you study, the more you will improve.

So the structure stays the same, while the verbs can change tense.

Not in the same way.

De plus en plus means more and more, which is related, but it is a different construction.

Compare:

• Plus je lis ce roman, plus je le trouve intéressant.
  = The more I read this novel, the more interesting I find it.

• Je trouve ce roman de plus en plus intéressant.
  = I find this novel more and more interesting.

These are close in meaning, but not identical in structure.

Your sentence emphasizes the relationship between two things: as my reading increases, my interest increases.

Yes. It is completely natural, standard French.

The plus..., plus... pattern is very common in both speech and writing. Your sentence sounds clear, idiomatic, and correct.