A legbook is just the legs of a book. Not a whole book. It only contains the parts that walk around and move, not so much the descriptions (things that don't move). This kind of book is a script or screenplay. These "books" are focused on actions and things people say, which is why it's just the legs.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
What the other scripts weren’t able to do, his script did.