Welcome to YT. Your mom's little ones are adorable. I think you'll love this forum and hopefully will learn a lot about yorkies. I think this is a very pleasant way to get information about the little furry ones. I can't give you much advice on books on yorkies in the States because I live in Spain but I have a few books myself and I don't believe it's necessary to buy lots of them because at the end of the day they are all a bit repetitive. Look around to see is there's a kind of journal or monthly magazine on Yorkies. These are normally written by a bunch of people and maybe one can get a broader view.
As somebody else said, stick around. I'm sure you'll learn a lot. The breeding section is one my favorite. It's quite useful.
Rubi