(علم الكلام) ilm-Al Kalam is one of the Major/famous sciences of Islam. It uses the brain/Aql(العقل) to look at the outer world and understand Allah, and it uses Naql(النقل) which is the use of Authentic Ahadith/Sayings of the Prophet (May Gods peace and blessings be upon Him). Ilmul-Kalam is used to fight against people of false beliefs and innovators, for example the Mu'tazila among many others. I only know the basic surface of what Ilmul-Kalam is (i.e what it is, its purpose, what it is used for, what scholars have said about it and so on).
My question is, what is the most Authentic book on Ilmul-Kala, that a beginner can start learning from? I know there are a number, I have searched the internet and have currently on my flash a book by (الشهرستاني), I do not want to know what the best book is, I want to know what the best and most reliable book is for a beginner.