A comprehensive explanation and commentary of the first 24 hadith of Imam al-Nawawi's classic compilation, Riyadh al-Saliheen.
Author: Abu Zakaria Al-Nawawi
The Three Fundamental Principles and the Four Basic Rules of Islam.
Author: Muhammad Bin Abdul Wahhab
Publisher: Daar Al-Watan
Source: http://www.islamhouse.com/p/1333
One of the most detailed books describing the rulings of Hajj, Umrah, and visiting the Masjid of the Prophet (peace be upon him).
Author: Sayid Sabiq
Source: http://www.islamhouse.com/p/1395
The "Islamic Studies" series of books by the prolific writer and scholar Dr. Bilal Philips are part of a meticulously written series intended to guide young Muslims in a manner that helps them acquire a thorough practical and spiritual foundation. This series have been generally acknowledged as an excellent introduction set to the principles of the Islamic faith, with core sciences discussed and explored, for both the new Muslim as well as the "old" Muslim wishing to brush up on the fundamentals.
Author: Abu Ameenah Bilal Philips
In this book, Dr. Saleh As-Saleh just mentions the Quranic verses talking about the following topics, for example: who is Allah, we should worship none but him, proofs for his existence, the final Messenger Muhammad (peace be upon him) and purpose of our life.
Author: Dr. Saleh As-Saleh
Publisher: http://understand-islam.net - Understand Islam Website
A nice discussion of some important points needed by those callling to Islam
Source: http://www.islamhouse.com/p/1243