Favorite book mentioned in the books:
I would probably have to say Fantastic Beasts and Where to Find Them by Newt Scamander. This mostly because I've always loved the magical beast in the Harry Potter series itself so knowing that someone went looking for them all is amazing. Not only did they find all of these beasts but they put together this book rating their dangerousness on a scale, it makes me really happy. Knowing also that I can read it, is great too.
And that little thing about it being made into a trilogy of movies penned by J.K. Rowling is something too... just saying.