How to Umpire (Including Knotty Problems) by Hall of Fame Umpire Billy Evans is available on-line at the Internet Archive. This classic is available in pdf and a few other formats.
This book, published in 1917, offers very practical advise to the modern umpire, such as:
Enjoy this terrific book from a umpire legend!
Never lose sight of the ball. If you know where it is at all times, you are not going to lose very many plays. It is worse for someone to pull the hidden ball trick and the umpire not to see it, than it is for the player who has been trapped.
Don't turn your head and look the other way after you have given a decision. A lot of things can happen while you are looking in the opposite direction.
Show a little "pepper" in your work. If the umpire is always on the alert, the players invariably get the spirit. If the umpire is content to have the game drag, the players seldom offer any objection.
A lot of people say umpires are not human. They are wrong. It is human to err, and umpires err. If they made as many mistakes as most fans insist they do, they would indeed be very human.