Learn Italian: Ha elogiato il mio libro, per di più ne sono molto felice. - He praised my book, furthermore I am very happy about it.

Here, ne is acting as a propositional pronoun. Instead of replacing a single noun (like "some of the books"), it replaces the entire preceding fact: "the fact that he praised my book". Because in Italian you are happy about something (essere felice di qualcosa), ne steps in to mean "about it" or "of this situation".

Could I just say di questo instead of ne here?

Yes, you could say ...per di più sono molto felice di questo ("furthermore I am very happy about this"). However, using the clitic pronoun ne is much more concise and makes your Italian sound fluid and natural, which is a great habit to build at the B2 level.

What does the phrase per di più add to the sentence?

Per di più translates to "furthermore," "moreover," or "what's more." It's a discourse marker used to add extra weight, emphasis, or a related follow-up point to the clause that was just stated.

Why is it il mio libro and not just mio libro?

As a quick reminder, Italian almost always requires a definite article (il, la, etc.) before a possessive adjective like mio. The main exception is when talking about singular close family members (like mio padre), but for everyday nouns like libro, you must include the article.