He found books of criticism on plays and playwriting, and he mastered them.

He found books that told how to construct plays, and he mastered them.
Frank Merriwell was a person with a vivid imagination and great mechanical and constructive ability.

Had this not been so, he might have studied forever and still never been able to write a successful play.

In him there was something study could not give, but study and effort brought it out.

He wrote a play.
"John Smith of Montana" was a success.