En la librería encontré una novela con el lomo rojo y una portada muy bonita.

No. Librería means bookshop / bookstore.

This is a very common false friend for English speakers:

• librería = bookstore
• biblioteca = library

So En la librería means In the bookshop or At the bookstore, not In the library.

Because en tells you where something happened.

Here, the speaker is saying the finding happened in/at the bookshop:

• En la librería encontré... = I found ... in the bookshop.

By contrast, a la librería usually suggests movement toward the place:

• Fui a la librería = I went to the bookshop.

So:

• en = location
• a = direction/destination

Encontré is the first-person singular preterite of encontrar.

That means:

• yo encontré = I found

The preterite is used because this is a completed action in the past: the speaker found the novel at a specific moment.

Compare:

• encontré = I found it
• encontraba = I was finding / I used to find / I kept finding

Also, the accent in encontré is important. It shows the stress falls on the last syllable: en-con-TRÉ.

Because novela is a feminine singular noun.

In Spanish, articles must agree with the noun:

• un libro
• una novela

You can often guess gender from the ending, and nouns ending in -a are often feminine, though not always. Here:

• novela → feminine
• therefore una novela

Here, lomo means the spine of a book.

So:

• el lomo rojo = the red spine

This is a useful word because lomo has different meanings depending on context. It can also mean things like:

• back
• loin
• ridge

But with books, lomo is specifically the spine.

This is about how Spanish uses articles when describing parts or features.

• el lomo rojo treats the spine as the specific spine that the book has
• una portada muy bonita presents the cover as a feature of the book, almost like a very pretty cover

Spanish often uses the definite article for body parts and parts of objects when they are understood as belonging to something already mentioned:

• un hombre con los ojos azules
• un perro con la cola larga
• un libro con el lomo rojo

With portada, the speaker chose una, which makes the description sound a bit more like one notable characteristic of the novel.

Could la portada also be possible? Yes, in many contexts it could. But the sentence as written sounds natural.

Because in Spanish, descriptive adjectives usually come after the noun.

So:

• lomo rojo = red spine
• portada bonita = pretty cover

This is the normal pattern in Spanish. English often puts adjectives before the noun, but Spanish usually puts them after.

Sometimes adjectives can go before the noun, but that often changes the tone, emphasis, or meaning. In this sentence, the standard post-noun position is the natural choice.

Because muy is used before adjectives and adverbs.

Here, bonita is an adjective, so:

• muy bonita = very pretty

Use:

• muy
  • adjective/adverb
    
    • muy bonita
    • muy interesante

But mucho is used differently, for example with nouns or as an adverb meaning a lot:

• mucho dinero = a lot of money
• leo mucho = I read a lot

So mucho bonita is incorrect.

Here, portada means the cover, especially the front cover or the visible designed cover of the book.

In everyday Spanish, portada is very natural when talking about how a book looks. Depending on context, you may also hear:

• cubierta = cover
• tapa = cover / hard cover, in some contexts

But in this sentence, una portada muy bonita clearly means the book had a very attractive cover.

Yes. Spanish word order is flexible.

You could say:

• En la librería encontré una novela...
• Encontré una novela... en la librería

Both are correct.

Starting with En la librería puts the location first, which can feel a little more like setting the scene:

• At the bookshop, I found...

So the original sentence is completely natural; it just gives early emphasis to where the action happened.