NEW EDITION for Parliamentarians, I recommend Robert's Rules of Order Newly Revised 12th ed.
NEW EDITION For members and chairs I recommend Robert's Rules of Order Newly Revised In Brief. This gives you the 20% of the rules that are used 80% of the time.
For a more in depth study of theory for the Parliamentarian, this book gives greater explanation of the reasoning and logic behind the different rules. This is great to help you apply the rules in unusual cases