According to Rabbi Chaim Y.D. Azulai, Rabbi Jacob and Rabbi Yair Chaim Bachrach the author of Chavos Yair, were the two greatest halachic authorities of their time.
Rabbi Jacobs major work is Shevus Yaakov, consisting of three collections of his responsa which deals with all areas of halacha and has remained a major source for halachic decisions until this day. He also wrote Minchas Yaakov, which supplements and seeks to reconcile contradictions in Rabbi Moshe Isserles Toras Chatas, and Chok Yaakov, an extensive commentary on the Orach Chaim portion of Shulchan Aruch.
However, only the portion on the Laws of Passover has been preserved, and appears in standard editions of the Shulchan Aruch. During a difficult period of his life Rabbi Jacob was unable to concentrate on his halachic studies. Instead he made a careful study of the Aggada of the Talmud and prepared a commentary, Iyun Yaakov, which appears in standard editions of the Ein Yaakov. Rabbi Jacob was a brother-in-law of two of the greatest rabbis of his time, Rabbi Dovid Oppenheim and Rabbi Eliyahu Shapiro, author of the Eliyahu Rabba, a well-known halachic work.
Sources: Orthodox Union