J’ai des livres, et je vais en lire deux ce soir.

Here, en is a pronoun that stands for des livres.

So instead of repeating livres, French uses en:

• J’ai des livres. Je vais lire deux livres ce soir.
• J’ai des livres, et je vais en lire deux ce soir.

In English, this is like saying I’m going to read two of them tonight.
So en often means something like of them, some, or any, depending on context.

Because deux by itself usually needs something to refer to.

In English, I’m going to read two sounds incomplete unless the context is very clear. French is similar. Since the second clause is referring back to des livres, French normally uses en to show that two means two of those books.

So:

• je vais en lire deux = natural
• je vais lire deux = usually feels unfinished or odd here

You could also repeat the noun:

• je vais lire deux livres ce soir

But if you do not repeat livres, then en is the normal replacement.

French object pronouns usually come before the verb they belong to.

With a simple verb:

• J’en lis deux.

With aller + infinitive:

• Je vais en lire deux.

Even though lire is the main action, the pronoun still goes before the infinitive lire, not after it.

So French says:

• je vais en lire deux

not:

• je vais lire en deux

That word order is a standard French pronoun pattern.

Because once the noun has been introduced, French often avoids repeating it.

First, the speaker introduces the category:

• J’ai des livres

Then the speaker refers back to that category with en and gives a quantity:

• je vais en lire deux

This is very common in French:

• Tu as des pommes ? J’en veux trois.
• Il a des idées, mais il n’en explique qu’une.

So the pattern is:

1. mention a noun
2. replace it with en
3. add a number or quantity

Because des is the normal plural indefinite article before a plural noun.

• un livre = a book
• des livres = some books / books

In J’ai des livres, the speaker is simply saying they have some books, without specifying which ones.

You often see de instead of des in other structures, but not here. For example:

• after negation: Je n’ai pas de livres
• after many quantity expressions: beaucoup de livres, deux livres

So:

• J’ai des livres = correct
• J’ai de livres = incorrect in this sentence

Both are possible, but they are not exactly the same in tone.

• je vais lire = near future, very common in everyday speech
• je lirai = simple future, also correct, sometimes a bit more formal or more neutral depending on context

In this sentence, ce soir makes the action feel close and planned, so je vais lire sounds very natural.

Compare:

• Ce soir, je vais en lire deux. = very natural in conversation
• Ce soir, j’en lirai deux. = also correct

French speakers often prefer aller + infinitive in casual speech.

Ce soir is a time expression meaning this evening / tonight.

French often uses ce directly with time words, without a preposition:

• ce matin
• cet après-midi
• ce soir
• cette semaine
• ce mois-ci

So French does not need a word like in or on here.

• ce soir = tonight / this evening

That is just the normal structure.

Because French normally contracts je before a vowel sound.

Since ai starts with a vowel, je becomes j’:

• j’ai
• j’aime
• j’habite

This is required in standard French.

So:

• J’ai = correct
• Je ai = incorrect

Yes, absolutely. That sentence is grammatically correct.

But using en is more natural when the noun has already been mentioned. It avoids repetition.

Compare:

• J’ai des livres, et je vais lire deux livres ce soir.
• J’ai des livres, et je vais en lire deux ce soir.

The second version sounds smoother and more typical because French often prefers pronouns instead of repeating the same noun.

En is mainly used to replace nouns introduced by de or by an indefinite/partitive idea such as:

• des livres → en
• du pain → en
• de l’eau → en
• beaucoup de films → en

Examples:

• Tu as des livres ? Oui, j’en ai.
• Il boit du café. Il en boit beaucoup.

It is not usually used the same way for a direct object with a definite article:

• Je vois les livres. Je les vois.
  not j’en vois, if you specifically mean the books

So in your sentence, en works because livres was introduced as des livres, not les livres.

Because deux is the quantity of the omitted noun, and in French that quantity normally comes after the verb when en is used.

The structure is:

• en + verb + quantity

Examples:

• J’en veux trois.
• Il en a beaucoup.
• Nous allons en acheter deux.

So:

• je vais en lire deux = standard order

The number stays after the verb, while en comes before it.

Not really. Both versions are acceptable:

• J’ai des livres et je vais en lire deux ce soir.
• J’ai des livres, et je vais en lire deux ce soir.

In French, a comma before et is often optional here. It may be used to mark a pause or make the sentence slightly clearer, but it is not required.

So this is more a punctuation choice than a grammar rule.