I am on baby #4 and I have always told my kids (and everyone else for that matter) immediately. It was too hard to walk around and not have it come up in front of them with my husband. I know most people wait to tell others until after the first trimester and I just never adhered to that either. I actually had a miscarriage after telling everyone too and was glad I had so many people to be there for me. I would not have known what to do if I couldn't talk to people.
As far as how I tell them, I just sit them down an tell them straight out. Kids understand more than you realize. In fact, they have always had lots of great questions that are fun to talk about with them. As far as books, I don't know of many but do have one that is called "My Little Brother" that shows little kids what it is like to be the big sister or brother.
Good luck and congrats on #3.