President Witherspoon, a two-volume biography of John Witherspoon published in 1925. It remains the best biography of Witherspoon out there. No other study is as comprehensive. Jeffry Morrison's John Witherspoon and the Founding of the American Republic (2007) focuses almost entirely on Witherspoon's political thought in the context of the Revolution (see my review of it here) and L. Gordon Tait's The Piety of John Witherspoon is more concerned with his religious life. Collins has written the only book I know that chronicles Witherspoon's life from beginning to end--from Scotland to Princeton.
When I read these old biographies I focus more on facts and evidence than I do on interpretation. Collins obviously likes Witherspoon, and the biography has several hagiographical moments, but works such as this remain valuable in my attempts at mastering the general flow of Witherspoon's life and imagining how I am going to piece these stories into my larger study. Because President Witherspoon is so meticulously researched and loaded with reprints of primary sources, I have been able to develop a long list of documents I will need to consult down the road.