Reddit mentions of Al-Ghazali's "Moderation in Belief"

Imam al-Ghazali's Moderation in Belief, tr. Aladdin M. Yaqub.

The book is essentially about proving Islam (and Sunni orthodoxy) on the basis of reason (as well as revelation), so it goes through an argument for the existence of God, and His attributes, and the relation of His attributes to His essence, it goes through various deviant sects of Islam and refutes their beliefs, it goes through proofs for the Prophethood of Muhammad (peace and blessings of God be upon him), and so on.

The translation I think is really good, at times Imam al-Ghazali is a bit unclear but the translator has excellent notes that expands upon the arguments made, and sometimes the translator formalizes some of the arguments al-Ghazali makes, making it much more understandable.

Do you mean like rational theology?

1. Cambridge Companion to Islamic Theology
   
   
2. Cambridge Companion to Arabic Philosophy
   
   
3. Moderation in Belief by Al-Ghazali. This is the famous Ash'arite (orthodox Islamic theology) thinker's defense of the school's doctrine. You will also be interested in his Incoherence of the Philosophers, the famous systematic refutation of avicennan philosophy (again if you're interested in these things).
   
   If, however, you want something of a more basic "introduction" to Islamic thought, which I'm guessing is what you actually want. You need to read the Qur'an. With a good commentary. I recommend nothing (and I mean nothing else- because I've read this one among others and this is the best intro for westerners) other than Muhammad Asad's "Message of the Qur'an". After this, you aught to read "Hadith: Muhammad's Legacy to the Medieval and Modern World". This is a great introduction to our "second scripture".
   
   Also, if you want a good ground-up introduction to Islamic theology you should check out this:
   
   https://islamtheologyscience.files.wordpress.com/2011/08/islamictheologyandsciencedraft.pdf
   
   Its written by one of our mods. Really good. But if rational theology/kalaam was not what you were looking for, then please read Muhammad Asad's quran commentary and Jonathan brown's book on Hadith that I listen above.