Complex sentence: Susan has inherited Death's perfect memory, which includes the ability to "remember the future" and an instinctive knowledge of the nature of Death's work, which gives her considerable assistance during her brief time doing her grandfather's job in "Soul Music".
Split into simpler sentences: Susan has inherited Death's perfect memory, which includes the ability to "remember the future" and an instinctive knowledge of the nature of Death's work. The latter gives her considerable assistance during her brief time doing her grandfather's work in "Soul Music".