John Immanuel Tremellius (1510-1580) was a Jewish convert to Christianity. From 1561 to 1577, he was professor of Old Testament at Heidelberg. He probably was involved with the production of the Heidelberg Catechism -- he is certainly responsible for the first translation of the Catechism into Hebrew.
I saw a notice for this book today in the most recent Sixteenth Century Journal:
From Judaism to Calvinism: The Life and Writings of Immanuel Tremellius (c.1510-1580), Kenneth Austin, Aldershot: Ashgate, 2007.
It's not cheap, but it looks fascinating!