التوضيح و البيان لشجرة الإيمان
للشيخ عبد الرحمن بن ناصر السعدي
« At-Tawdîh wal-Bayân li-shajarat Al-Imân », by Shaykh As-Sa'di, is an excellent book on Faith.
The author defines faith and explain it, he mentions the causes that allow to increase it and also its benefits and fruits on the individual.
2 editions available:
مكتبة دار المنهاج - Beautiful hard cover edition whose text has been transcribed from a manuscript.
دار المنهاج - Softcover.