John Calvin is one of the greatest theologians and Bible commentators the church has ever known. I have used his commentaries on the Psalms for study and found them extremely insightful, readable, and fair to the text. I bought the first edition of his commentary on the Psalms for my Nook for just a few dollars. Here is a link to the Kindle editions of his commentaries on Amazon. Here is the same resource in the Nook store.