Tuhfah As-Saniyyah (a rather popular explanation of al-Ajuroomiyyah).  The author is Muhammad Muhy ad-Deen `Abd al-Hameed and his book is the best explanation of al-Ajuroomiyyah I have yet to see.  However, it isn’t the most basic, which is why you find that this explanation itself, has been ‘explained’. Printed by al-Maktabah al-'Asiriyyah (Beirut) printed on white paper 160 pages.