El catálogo y el cancionero son muy largos.

In Spanish, it is normal to repeat the article before each singular noun in a list:

el catálogo y el cancionero

This makes it clear that you are talking about two separate things.
Leaving out the second el would sound unnatural here.

A good basic rule is: if you have two separate singular nouns joined by y, you will often repeat the article.

Because the subject is plural.

The subject is:

el catálogo y el cancionero = the catalogue and the songbook

That means two things, so Spanish uses the plural form of ser:

• es = is
• son = are

So:

• El catálogo es muy largo. = The catalogue is very long.
• El catálogo y el cancionero son muy largos. = The catalogue and the songbook are very long.

Because adjectives in Spanish usually agree with the noun in number and gender.

Here, the two nouns together form a plural masculine subject:

• catálogo = masculine singular
• cancionero = masculine singular
• together = masculine plural

So the adjective becomes:

• largo = singular masculine
• largos = plural masculine

That is why the sentence uses muy largos.

Because both nouns are masculine:

• el catálogo
• el cancionero

When two masculine nouns are joined, the adjective is masculine plural:

largos

If the nouns were feminine, you would use largas.

For example:

• La revista y la novela son muy largas.

In Spanish, muy means very, and it normally goes directly before an adjective or adverb:

• muy largo = very long
• muy altos = very tall
• muy interesante = very interesting

So:

son muy largos = they are very long

This is the normal word order.

Yes. The article el shows that both nouns are masculine singular.

• el catálogo
• el cancionero

In Spanish, nouns have grammatical gender, and the article usually tells you the gender:

• el = masculine singular
• la = feminine singular
• los = masculine plural
• las = feminine plural

So seeing el before both nouns tells you they are masculine.

Y means and.

It joins the two nouns:

el catálogo y el cancionero

So it tells you that both things are part of the subject.

A useful thing to remember: when y joins two singular nouns, the whole subject usually becomes plural, which is why the sentence uses son and largos.

The accent mark shows where the stress goes in pronunciation.

catálogo is stressed on the syllable tá:

ca-TÁ-lo-go

Without the written accent, Spanish spelling rules would suggest a different stress pattern, so the accent mark is needed.

This is a pronunciation/spelling feature, not a change in meaning.

No. Cancionero follows normal Spanish stress rules, so it does not need a written accent.

It is pronounced:

can-cio-NE-ro

In Spanish, words ending in a vowel, n, or s are normally stressed on the second-to-last syllable. Since cancionero already follows that pattern, no accent mark is needed.

Spanish uses ser for general characteristics or descriptions, and estar more for states, conditions, or location.

Here, long is being treated as a general descriptive quality of the catalogue and the songbook, so ser is the natural choice:

son muy largos

Using estar here would usually sound odd unless you were talking about something more temporary or context-based, which is not the normal interpretation.

Yes. That would still be correct.

• son largos = they are long
• son muy largos = they are very long

Muy just adds extra emphasis.

You would change the verb and adjective to singular:

• El catálogo es muy largo.
• El cancionero es muy largo.

Compare:

• es / largo for one masculine singular noun
• son / largos for two masculine nouns together