Here you can find ebooks and articles about the meaning of La ilaha illa Allah, and what it contains of issues such as: Al-Kufr bit-Taghut, declaring takfir upon the mushrikun, the pillars and conditions of Tawhid and more.
For a summarized overview of important issues on Tawhid refer to the book “An introduction to Tawhid and Sunnah”.
UPDATED!! Some issues regarding takfir
Mufid Al-Mustafid fi Kufr Tarik At-Tawhid – by Muhammad ibn Abdul-Wahhab
The Six Principles – by Muhammad ibn Abdul-Wahhab
The Four Principles – by Muhammad ibn Abdul-Wahhab
Explanation of six places in the Sirah – by Muhammad ibn Abdul-Wahhab
Takfir upon the mushrikun and its relation to Kufr bit-Taghut – by Ahmad ibn ‘Umar Al-Hazimi