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
A nice discussion of some important points needed by those callling to Islam
Source: http://www.islamhouse.com/p/1243
A discussion of the 'pillars' of Eemaan and the factors which invalidate ISlam and remove a person from its fold.
Author: Abdul Aziz bin Abdullah bin Baz
Source: http://www.islamhouse.com/p/1227
A useful booklet shows 1000 acts of Sunnah of the Prophet Muhammad (peace be upon him). It contains different situations i.e. waking up, going to the bathroom, performing ablutions, using sewak, wearing shoes, wearing clothes, going out and coming back home, going to the mosque, athan, iqama, praying at night and post-prayer sunnahs.
Author: Khaled Al Husainan - Khaled Al-Husainan
Publisher: http://www.rasoulallah.net - Website of Rasoulullah (peace be upon him)
This is a useful book although it is small. It contains some technical terms for da'wah in English. No doubt all scholars and callers to Islam in English are in much need to something like that to conveys the right message of Islam to all those speak English.
Brief guide concerning the fundamentals of Islam.
Author: Naasir Bin Abdulkarim al-Aql
Source: http://www.islamhouse.com/p/1295