As a person that is trying to know more about Islam, you are advised to read the following books that explain the Islamic beliefs, including the concept of Divine Oneness. Here are a few titles, some of which are a little more scholarly than the others.
1) Towards Understanding Islam by Maududi
2) Islam in Focus by Hammudah Abdul Ati
3) A Complete Idiot’s Guide to Islam by Yahya Emerick
3) Tawhid by Dr. Isma’il R. Faruqi.
The first three books explains the Islamic beliefs and practices. As for the last mentioned one, it is a somewhat more philosophical exposition of the Islamic monotheism, and shows how it is the most rational, and therefore the most appealing to reason and common sense.