Ŝi ĝuas la libron.

Ŝi means she.

The letter ŝ is a separate Esperanto letter, not just s with an accent. It is pronounced like sh in she.

So:

• ŝi = she
• si = a different word, a reflexive pronoun meaning oneself / himself / herself / itself / themselves in certain contexts

That means ŝi and si are not interchangeable.

A simple pronunciation guide is:

• ŝi ≈ shee
• ĝuas ≈ JOO-ahs
• la ≈ lah
• libron ≈ LEE-bron

A few helpful points:

• ŝ sounds like sh
• ĝ sounds like the j in jam
• u sounds like oo
• Stress in Esperanto normally falls on the second-to-last syllable

So the stress is:

• Ŝi
• ĜU-as
• la
• LI-bron

In Esperanto, -as marks the present tense.

So:

• ĝui = to enjoy
• ĝuas = enjoys / is enjoying

This ending does not change for different persons:

• mi ĝuas = I enjoy
• vi ĝuas = you enjoy
• ŝi ĝuas = she enjoys
• ili ĝuas = they enjoy

Unlike English, the verb form stays the same.

The dictionary form is ĝui, which means to enjoy.

Esperanto verbs are usually learned in the infinitive form ending in -i:

• ĝui = to enjoy
• legi = to read
• esti = to be

Then you change the ending for tense:

• ĝui = to enjoy
• ĝuas = enjoys / is enjoying
• ĝuis = enjoyed
• ĝuos = will enjoy

The -n ending marks the direct object.

Here, the book is the thing being enjoyed, so it takes -n:

• libro = book
• libron = book as a direct object

Compare:

• La libro estas interesa. = The book is interesting.
  Here libro is the subject.
• Ŝi ĝuas la libron. = She enjoys the book.
  Here libron is the object.

This -n ending is one of the most important features of Esperanto grammar.

Because la never changes.

In Esperanto:

• la = the
• it is used for singular and plural
• it does not change for case
• it does not change for gender

So all of these use the same article:

• la libro = the book
• la libron = the book (object form)
• la libroj = the books
• la librojn = the books (object form)

The noun changes, but la does not.

No. Esperanto has la for the, but it has no indefinite article equivalent to English a/an.

So:

• libro can mean a book or just book, depending on context
• la libro means the book

If the sentence were Ŝi ĝuas libron, that would usually mean She is enjoying a book.

Yes, Esperanto word order is fairly flexible, mainly because the -n ending shows which noun is the object.

So all of these are grammatical:

• Ŝi ĝuas la libron.
• La libron ŝi ĝuas.
• La libron ĝuas ŝi.

However, the most neutral and common order is subject–verb–object, so Ŝi ĝuas la libron is the standard way to say it.

Changing the order can add emphasis or sound more literary.

Usually yes, but it is good to think of ĝui as take pleasure in / enjoy.

It can be used with things, activities, and experiences:

• ĝui la libron = enjoy the book
• ĝui la muzikon = enjoy the music
• ĝui la feriojn = enjoy the holidays

In many contexts it matches English enjoy very closely.

Normally, no. Esperanto usually keeps the subject pronoun.

So you would say:

• Ŝi ĝuas la libron.

not normally just:

• Ĝuas la libron.

The verb ending -as only tells you the tense, not the person, so the pronoun is usually needed unless the subject is already clearly expressed elsewhere in the sentence.

You would make book plural and keep the object ending:

• libro = book
• libroj = books
• libron = book (object)
• librojn = books (object)

So:

• Ŝi ĝuas la librojn. = She enjoys the books.

This shows how Esperanto stacks endings in a very regular way:

• -j = plural
• -n = direct object

So librojn = book + plural + object.