Catholic Online: St. Zosimus: Brief biography says that the pope was so disliked in Rome that his death was the occasion of celebration in the streets.
Patron Saints Index: Zosimus: Profile mentions a few surviving documents from his pontificate.
Pope St. Zosimus: Account of his pontificate focuses on his entanglement with the Pelagian controversy.
Popes Through the Ages: St. Zosimus: Overview of his pontificate. From the book by Joseph Brusher.
The Ecole Glossary: Zosimus: Brief biographical profile, by Karen Rae Keck.
Zozimus (Zosimus), Pope: Says that he formally condemned Pelagianism after initially being misled by its proponents.