Understanding Islam requires, at minimum, reading one of the Qur'an translations or tafsirs and one of the two sahih hadith collections.  One should also sample Islamic law: Reliance Of The Traveller, Book o, Chapter  9, Risala Ch. 30 or Hedaya, vol. 2, pages 139-218.  For the short cours, inadequate but better than abject ignorance read the Jihad chapter of Riyud us-Saliheen.