What is a 50 page book called?

Excuse me, could you please clarify for me? I'm a bit confused by your question. Are you asking about the title of a specific 50-page book, or are you wondering about a general term that could be used to describe any 50-page book? If it's the latter, I'm not aware of a specific term in common usage that solely refers to a 50-page book. A book's title is unique to that particular work, regardless of its length. So, unless you're referring to a specific book, there isn't a universal term for a 50-page book. Could you please provide more context or clarify your question?