If you are looking for "free" resources for information on mechanics, I would recommend
http://www.umpire.org That site has articles and mechanics section that contain good information to start. Also if you go to their Message Board and then enter keywords to search for threads you might be to find some good information. Another good resource is
http://www.amateurumpire.com
If you want to pay, I would recommend Carl Childress' book Working the Bases/Working the Plate. The book is available on the Gerry Davis website [url]http://www.gerrydavis.com[/url)and covers everything you need to get started.
Good luck.