"Il est l'homme central dans le livre."
"He's the main character in the book" - is that what it really mean?
No. While "main character" is heard and used much more often than "main man", the translation is right. In this case they are referring specifically to the main male character in the book, who may or may not be the main character. In Sleeping Beauty, the prince was the main male character, but not the main character (that was Sleeping Beauty). "Main character" would be along the lines of "le personnage principal du livre". Hope that helps!