I am new to currency collecting but have been a coin collector over 30 years now. Recently I purchased a collection that contains a large number of US currency dollars, about 110 along with the coins. There are large and small size bills. Mainly silver certificates and 2 gold certificates. What I need to get is a book or books like they have for coins. I mean like the red and blue coin books. Do they make anything like that for currency? If not where can I find the prices they are worth and how to grade them. I'm going crazy trying to follow values on ebay. The $10 and $20 gold certificates I got down pretty well. And please tell me what a "Mule" is! Any help will be highly appreciated. Thank you.