Learn Hardcore Esperanto: Li havas libron.

Esperanto does not use an indefinite article at all. When you see libron in Esperanto, you can translate it into English as a book, one book, or simply book, depending on context.

What does the -n ending on libron signify?

In Esperanto, nouns end in -o, and when a noun is the direct object of a verb, it takes the -n ending. Since libron is receiving the action of havas (has), it becomes libron rather than libro.

Why does the verb end in -as?

All Esperanto verbs in the present tense end in -as. So havas is the present-tense form of the verb meaning to have.

What does li mean, and can it ever change form?

Li means he. Pronouns in Esperanto do not change form based on grammatical case, so li stays the same whether it is the subject or something else in the sentence. The only time you’d add the -n ending to a pronoun is in certain standalone cases (like in short replies), but generally pronouns remain unchanged in normal word order.