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.

at-Tuhfatus Sanniyyah التفحة السنية