Johns himself left little in the way of personal papers, but the Sheridan Libraries do hold a small collection titled The Hopkins Family collection.

The Alan Mason Chesney Medical Archives also maintains a small collection of family papers, the Johns Hopkins Collection.

The Johns Hopkins Biographical Archive features a selection of digitized primary sources about Johns, including evidence that he owned enslaved people.

Biographical dictionary compilations also include articles on Johns. One of the more recent ones is in American National Biography. 

Johns Hopkins: A Silhouette was written in 1929 (repr. 2009) by Helen Hopkins Thom, a grand-niece of the founder. The work is based on anecdotes and reminiscences by her parents, aunts and uncle. It includes many unsubstantiated claims and inaccuracies, including the now-debunked assertion that Johns Hopkins did not own enslaved people. 

A biographical article, “Mr. Johns Hopkins,” was published in the Johns Hopkins Magazine in January 1974. A brief account of the founder's life, it relies heavily on Thom's book and therefore should be read with a critical eye.

Another good start for learning more about Johns Hopkins the man is his Wikipedia entry.