"Kitab At-Tawhid El Monoteismo" (Kitab At-Tauhid) is written in the spanish language. Kitab At-Tauhid is one of the best books on the subject of Tawheed (Tauheed, Tawhid - Monotheism) and ranks high in genuineness. All the relevant verses of the Quran have been discussed reasonably, rationally, and sincerely in this book. The essence of the Quran and Sunnah is placed in a very simple manner in this book.
Other Sources:
Quran verses on the prerequisites to get Allah's mercy