This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Your nomenclature is not quite right. Start in the Java tutorials. And, yes, there are great advantages to nested classes; we can have classes which are used only by another class and give them local scope or private access.