Risaalah fil-Ijtihaad - Mulla Khusru

This is a treatise on ijtihaad (Independent interpretive judgment), short but great in prospects and meanings, written by the Sheikh al-Islam in the Ottoman Empire, scholar Mulla Khusraw (d. 885 AH), at the request of his student, the caliph of the Muslims in his era, Sultan Muhammad al-Fatih (d. 886 AH). 

The letter was verified on the author's manuscript with his own handwriting.