The most comprehensive work of Weil is his Geschichte der Chalifen (5 vols., Heidelberg and Stuttgart, 1846–51), which is virtually an elaboration of the original works of Muslim historians, whom he in large part studied from manuscripts; it treats also of the Egyptian and Spanish califates. This was followed by the Geschichte der Islamischen Völker von Mohammed bis zur Zeit des Sultans Selim. (Stuttgart, 1866), an introduction to the medieval history of the Orient.
Weil's most significant work is his five-volume "History of the Caliphs," which he based on earlier Muslim historians' works and manuscripts. It covers the history of the caliphates in Egypt and Spain, as well as the broader history of Islamic peoples from the Prophet Mohammed to Sultan Selim.