The Meaning of Worship - Shaykh ’Abdur-Rahmaan Ibn Hasan aalush-Shaykh Taken from the explanation of Kitaabut-Tawheed – the monumental work of Shaykhul Islaam Muhammad Ibn ’Abdul-Wahhaab (d.1206) – rahimahullaahu ta’aalaa – it will serve as an excellent introduction.
Fiqhul 'Ibaadaat (Understanding Worship) - Q & A format - 178 pages - understand-islam.net [PDF] Shaykh Muhammad Saleh Al Uthaymeen (rahimahullaah)-Rendered into English by Dr. Saleh As-Saleh