One of the most entertaining things you can do is read the correspondence of the Reformers. They let fly all their true feelings about one another with aplomb and distinction. Their formal works are more restrained (imagine that!) than their private letters. For instance, publicly Calvin was always respectful towards Luther. Privately, with Bucer, he freely spoke his mind:
We learn from a letter to Bucer, dated January 12, 1538, that Calvin, amid all these movements in Geneva, sometimes employed his attention on Germany. In the writing referred to, he complains of Luther’s untractable nature, guards himself against his doctrine of the Lord’s Supper, and speaks with a force and decision which plainly indicate that he never inclined to the opinions of the Lutherans. It is not clear what gave rise to this complaint, since Luther, in 1537, directed a friendly address to the burgomaster of Basel, and a very conciliatory one also, dated Dec. 1st of the same year, to the reformed Swiss cantons, congratulating them on the concord to which they had attained.*
Calvin’s letter is in his Opera Omnia x/2, Letter 87. And it’s long. Calvin was in a mood on the 12th of January, 1538.
*The Life and Times of John Calvin, the Great Reformer (Vol. 1, p. 123).