For me, "The Big Bam" would count as a definitive for Ruth. I have not yet read Creamer's book but have heard it's great.
I heartily agree with you on the Johnson bio. That book was fantastic.
As for Wagner, "Honus Wagner: The Life of Baseball's "Flying Dutchman"" by Hittner is regarded as great.
For Mathewson, I would go with "Christy Mathewson, the Christian Gentleman: How One Man's Faith and Fastball Forever Changed Baseball" by Gaines.
We will agree to disagree on "batsman"
It's an antiquated term that sticks out when used in modern prose, IMHO.
Don't let my couple of niggles on Hornbaker's book dissuade you from reading it. It really is terrific. In general, the book is well-edited and "tight" but those couple things did slip through.