The Book Of Monotheism

Kitab (Book) at-Tawheed (Monotheism) by Shaykh-ul-Islam Muhammad Ibn Abdul-Wahab.
The book covers a lot of major issues, with evidence from the Qur'an and the Sunnah.
A must read for every Muslim to understand their religion better - Knowledge is religion, so take your knowledge from those who are upon the Salaf manhaj.

Muhammad ibn ʿAbd al-Wahhab (Arabic: محمد بن عبد الوهاب‎; 1703 – 22 June 1792) was an Arabian Islamic Salafi[citation needed] scholar. His pact with Muhammad bin Saud helped to establish the first Saudi state and began a dynastic alliance and power-sharing arrangement between their families which continues to the present day. The descendants of Ibn ʿAbd al-Wahhab, the Al ash-Sheikh, have historically led the ulama in the Saudi state, dominating the state's clerical institutions. - Wiki.