El libro es delgado.

Because Spanish normally uses an article with a specific singular noun in a full sentence.

• el = the
• libro = book

So El libro means the book.

Saying just Libro es delgado would sound incomplete or ungrammatical in normal Spanish.

In Spanish, nouns have grammatical gender, even when they refer to things.

Libro is a masculine noun, so it takes:

• el for the
• masculine adjective forms like delgado

A common pattern is that many nouns ending in -o are masculine, and libro follows that pattern.

Es comes from ser, and está comes from estar.

Here, es is used because delgado describes a characteristic of the book: its thickness. That is treated as an inherent quality, not a temporary condition.

So:

• El libro es delgado = the book is thin

For a book, está delgado would sound wrong or very unnatural.

Because adjectives in Spanish usually agree with the noun they describe.

Since libro is:

• masculine
• singular

the adjective must also be:

• masculine
• singular

So we use delgado.

Other forms would be:

• delgada for feminine singular
• delgados for masculine plural
• delgadas for feminine plural

For example:

• El libro es delgado
• La revista es delgada
• Los libros son delgados

Very often, yes.

When an adjective directly describes a noun, it usually comes after it:

• un libro delgado

In your sentence, the adjective comes after the verb es:

• El libro es delgado

That is completely normal.

Putting delgado before libro would be unusual here and would sound literary or marked rather than neutral.

Yes. Delgado can describe something physically thin, including objects.

So El libro es delgado is grammatically correct and understandable.

Depending on context, Spanish can also use other words for physical thinness, such as fino, but delgado is a normal and clear choice.

A simple English-friendly guide is:

el LEE-bro es del-GAH-do

A few useful notes:

• libro has the stress on LI
• delgado has the stress on GA
• the r in libro is a light Spanish r, not a strong English r
• in Spain, the d in the middle of delgado is usually softer than an English d

You would make both the noun phrase and the verb plural, and the adjective would also agree:

• El libro es delgado = singular
• Los libros son delgados = plural

Changes:

• el → los
• libro → libros
• es → son
• delgado → delgados

This shows how Spanish matches articles, nouns, verbs, and adjectives.