Al-Mustasfa min 'ilm al-usul. (On Legal theory of Muslim Jurisprudence) is Imam Ghazali’s work on the subject of Usul Al Fiqh. It is considered as one of the four great works in the subject. The other three being,
Ghazali’s approach to usul al-fiqh , as articulated in this last and greatest work of Law, al-Mustafa , is based on the premise that, in essence, this science is knowledge of how to extract ahkam (rules) from the Shari’ah sources. (As for the science of fiqh , it concerns itself particularly with the Shari’ah rules themselves which have been established in order to qualify the acts of the locus of obligation, man.) Accordingly, Ghazali views it as imperative that any discourse on usul focus on three essential the ahkam ; the adilla (sources); and the means by which rules are extracted from these sources, which ultimately includes examination of the qualifications of the extractor, namely, the mujtahid .
An exemplary exposition by one of the most eminent scholars of the Islamic tradition on the principles of Jurisprudence (usul al fiqh). Quite a dense read but worth it for those that are interested in this rich subject.