Individual Geonim produced valuable works (of which later), but what is perhaps most important from the point of view of the development of Judaism is the literature of their Responsa or answers to questions, chiefly on halakhic matters, addressed to them from various countries.
Besides the Responsa, but closely related to them, we have the lesser Halakhoth of Yehudai Gaon of Sura (8th century) and the great Halakhoth of Simeon Qayyara of Sura (not Gaon) in the 9th century.
The latter was a prolific writer of great influence, chiefly known for his Responsa, but also for his halakhic treatiseE, hiddushin and tosaphot.h.
Mordecai was also the author of Responsa.
He was author of many "Responsa," but his chief work was liturgical.