Learn Portuguese (Portugal): Este livro é para mim.

The pronoun me is used when it is the direct object of a verb. But here, the pronoun comes immediately after the preposition para (for). In Portuguese, prepositions must be followed by a special set of pronouns, so "me" changes to mim.

Why do we use é (from the verb ser) instead of está (from estar) for "is"?

We use é because the book's destination or ownership (who it is for) is treated as a permanent fact or characteristic, not a temporary state or location.

What does este tell us about the book's location?

Este means "this" and is specifically used for things that are physically close to the person speaking. The speaker is probably holding the book or has it right next to them.

If the object were a pen (a caneta) instead of a book (o livro), how would the sentence change?

It would become "Esta caneta é para mim". The word for "this" must match the gender of the noun, so the masculine este becomes the feminine esta. The word mim stays exactly the same, as it simply refers to "me" regardless of gender.