En la librería hay novelas de misterio; muchas son fáciles de leer.

No. Librería means bookshop/bookstore, not library.

This is a very common false friend for English speakers:

• librería = bookshop / bookstore
• biblioteca = library

So En la librería means In the bookshop or At the bookstore.

En usually means in, at, or sometimes inside depending on context.

Here, En la librería means:

• In the bookshop
• or At the bookstore

It tells you the location where the novels are. Spanish uses en very naturally for this kind of place expression.

Hay means there is or there are.

So:

• hay una novela = there is a novel
• hay novelas = there are novels

Spanish uses hay to say that something exists or is present somewhere.

Why not son or están?

• ser (son) is used to say what something is
• estar (están) is used to say where specific things are or what state they are in
• haber (hay) is used to introduce the existence of things

So in this sentence, the idea is not the novels are located... but there are novels...

Because hay is an impersonal form of haber. In modern standard Spanish, it does not change for singular or plural things in the present tense.

So both are correct:

• Hay una novela.
• Hay novelas.

English changes from there is to there are, but Spanish just uses hay for both.

After hay, Spanish often uses a noun with no article when introducing things in a general, indefinite way.

So:

• hay novelas de misterio = there are mystery novels

This means some mystery novels exist there; it does not mean specific, already identified novels.

That is why novelas de misterio appears without las here.

Literally, de misterio means of mystery, but in natural English it means mystery as a type or genre.

So:

• novelas de misterio = mystery novels

Spanish often uses de + noun to classify something:

• novela de aventuras = adventure novel
• película de terror = horror film
• libro de cocina = cookbook / cooking book

Because muchas refers back to novelas, which is feminine plural.

Agreement:

• novela = feminine singular
• novelas = feminine plural
• so muchas must also be feminine plural

If the noun were masculine, you would use muchos:

• muchos libros

Because muchas is being used as a pronoun here, meaning many of them.

The full idea is:

• muchas novelas son fáciles de leer

But Spanish often leaves out the noun when it is already clear from context:

• En la librería hay novelas de misterio; muchas son fáciles de leer.
• = In the bookshop there are mystery novels; many are easy to read.

So muchas stands for many of the novels.

Because fáciles de leer describes a characteristic of the novels, not a temporary condition.

Spanish usually uses:

• ser for descriptions, qualities, classifications
• estar for states, conditions, locations

Here, easy to read is being treated as a general quality of many of those novels, so son is the normal choice:

• muchas son fáciles de leer

Using están here would sound unnatural in standard usage.

Because the adjective has to agree with the plural subject muchas.

So:

• una novela es fácil
• muchas novelas son fáciles

The adjective changes to plural:

• fácil → fáciles

The singular form is fácil, with the stress on the first syllable.

When it becomes plural, Spanish keeps the stress in the same place:

• fácil
• fáciles

The accent mark shows that the stress still falls on fá-.

Without the accent, the word would follow normal spelling stress rules and be pronounced differently.

After adjectives like fácil and difícil, Spanish often uses de + infinitive to mean easy/difficult to do something.

So:

• fácil de leer = easy to read
• difícil de entender = difficult to understand
• fácil de usar = easy to use

This is a very common pattern in Spanish.

The semicolon links two closely related complete ideas:

• En la librería hay novelas de misterio
• muchas son fáciles de leer

It is stronger than a comma but not as separate as a full stop. In English, the same punctuation choice would also be possible.

A full stop would also work:

• En la librería hay novelas de misterio. Muchas son fáciles de leer.

The semicolon just shows the ideas are closely connected.