Reddit reviews Free Will: A Very Short Introduction

I think S. Blackburn's Think is an excellent introduction to some of the major areas in philosophy. You might also what to look at some of the philosophical books in the "Very Short Introduction" series, for example the Philosophy, Metaphysics, Ethics, Philosophy of Science and Free Will ones, which as you can guess are good places to start.

A book I quite enjoyed as an introduction to the great philosophers was The Philosophy Book, which not only gave clear descriptions of each of the philosophers' views, but also often gave a clear flowchart summary of their arguments.

On free will specifically? Start with the SEP to get a basis. Then for specific books and examples I'd recommend:

• Philsophy of Mind: A Comprehensive Introduction
  
  
• God, Freedom, and Evil
  
  
• Free Will: A Very Short Introduction
  
  
• On Grace and Free Will
  
  
• Summa Theologiae: Prima Paris, Q 83