Я люблю книги, у которых крепкий переплёт и аккуратный корешок.

Книги is the accusative plural of книга.

The verb любить normally takes a direct object in the accusative case:

• Я люблю книгу = I love a book
• Я люблю книги = I love books

For inanimate plural nouns like книги, the accusative plural looks the same as the nominative plural.

Because Russian is expressing the idea books that have... by using the structure у кого/чего есть... or, in a relative clause, у которых...

Literally, книги, у которых крепкий переплёт... is something like:

• books, by which there is a sturdy binding...

But in natural English, that means:

• books that have a sturdy binding...

If you said книги, которые крепкий переплёт..., that would be ungrammatical. The relative pronoun needs to fit the structure of the clause, and here the clause is built with у.

Которых is in the genitive plural.

That is because it follows the preposition у, which requires the genitive:

• у книги
• у книг
• у которых

Since книги is plural, the relative pronoun also has to be plural:

• singular: книга, у которой...
• plural: книги, у которых...

It is a relative clause describing книги.

Structure:

• книги = books
• у которых = which have / whose
• крепкий переплёт = a sturdy binding
• и аккуратный корешок = and a neat spine

So the clause modifies книги and tells you what kind of books the speaker likes.

A very literal breakdown is:

• I love books, at which there is a sturdy binding and a neat spine.

Natural English:

• I love books that have a sturdy binding and a neat spine.

Because they do not describe книги. They describe:

• переплёт — masculine singular
• корешок — masculine singular

So the agreement is:

• крепкий переплёт
• аккуратный корешок

If the adjectives described книги, then they would be plural:

• интересные книги
• старые книги

But here the books are being described indirectly through the things they have.

Russian often uses the singular for parts or features that each item has individually.

So:

• книги, у которых крепкий переплёт и аккуратный корешок

means that each book has:

• one binding
• one spine

English does this too:

• I like houses with a red roof not necessarily
• with red roofs

Both languages often use the singular when referring to a characteristic of each member of a group.

Not exactly.

• переплёт = binding; the whole bound structure of the book, especially a sturdy or hard binding
• обложка = cover, especially the outer cover
• корешок = spine of the book

So крепкий переплёт suggests a book that is solidly bound, not just one with a nice cover.

Корешок usually means the spine of a book here.

Yes, it does look like a diminutive form, and historically it is related to корень (root). In modern usage, though, корешок is a normal everyday word for a book’s spine.

So in this sentence:

• аккуратный корешок = a neat/tidy spine

It does not sound childish or unusually small here; it is just the standard word.

Because у которых крепкий переплёт и аккуратный корешок is a relative clause that adds information about книги.

Russian normally separates this kind of clause with a comma:

• Я люблю книги, у которых...

This is similar to how English often separates relative clauses in writing, although punctuation rules are not exactly the same in the two languages.

Yes. A very natural alternative is:

• Я люблю книги с крепким переплётом и аккуратным корешком.

This means:

• I love books with a sturdy binding and a neat spine.

This version is often simpler and more compact.

The version with у которых is also correct, but it feels a bit more explicit:

• books which have...

The version with с feels more like:

• books with...

It can correspond to either, depending on what sounds natural in English.

For example:

• книги, у которых крепкий переплёт
  could be translated as
• books that have a sturdy binding or
• books whose binding is sturdy

In this sentence, that have is probably the most natural English translation. But grammatically, the Russian structure is often close to an English whose idea.

Not completely. Russian word order is fairly flexible, though some orders sound more natural than others.

The given sentence:

• Я люблю книги, у которых крепкий переплёт и аккуратный корешок.

is neutral and natural.

You could move things around for emphasis, for example:

• Книги, у которых крепкий переплёт и аккуратный корешок, я люблю.

This puts extra focus on книги. It is grammatical, but less neutral.

So the original version is a good standard model to learn.

Yes, you can:

• Я люблю книги, у которых есть крепкий переплёт и аккуратный корешок.

This is grammatical and clear. The word есть emphasizes the idea of having.

However, in many sentences like this, Russian often leaves есть out when it is not needed. So the original version sounds smooth and natural:

• у которых крепкий переплёт и аккуратный корешок

Both are correct; the version without есть is simply more concise.