Zacharius Ursinus - Biographical article in New Schaff-Herzog Encyclopedia of Religious Knowledge. - http://www.ccel.org/ccel/schaff/encyc12/Page_111.html
What is The Gospel? - Online book by Ursinus. - http://www.ccel.org/ccel/ursinus/gospel.toc.html
Justification by Grace Alone Through Faith Alone - From his Commentary on the Heidelberg Catechism, specifically Questions 59 and 60. - http://www.truecovenanter.com/gospel/ursinus_justification.html
Ursinus and Olevianus - Biographical articles on both reformers. - http://www.prca.org/books/portraits/ursinus.htm
Ursinus on Worship - From Ursinus' Commentary on the Heidelberg Catechism, specifically on Question 96. - http://www.apuritansmind.com/PuritanWorship/ZachariasUrsinusOnWorship.htm
Catechisms of Ursinus - The large and small catechisms written by Ursinus compared in a three column format with the Heidelberg Catechism, written by Ursinus and Olevianus. - http://www.christreformed.org/doctrinevision/ursinus_project.pdf