Cottret does give an abundance of interesting historical and biographical detail, his description of the early years of reform in France (e.g., the affair of the Placards, 1534) is wonderful, and his portrait of Geneva is fascinating (Parts I and II of the book). But when it comes to Calvin's theology (Part III), he does a shockingly poor job.
In treating Calvin's theology, Cottret deals first, and at rather greater length, with Calvin's polemical works and sermons (chapters 12-13), and only then does he turn to a brief analysis of the Institutes (chapter 14). Cottret thus gives to an apparently random sampling of Calvin's occasional pieces (especially the treatise On Scandals, 1550) greater interpretive weight for Calvin as a theologian than to Calvin's life-work of systematic theology. This is absurd. What's worse, we get no real consideration of Calvin's theology as expressed in his commentaries. Does Cottret think that, because he is portraying 'a historian's Calvin' (p. x), and not a theologian's, he can simply ignore this source? What's still worse, when Cottret does finally get to the Institutes, he totally arbitrarily, without explanation, and against the entire consensus of Calvin scholarship, selects as his basis of exposition the 1541 French edition as 'the most significant version during the Reformer's lifetime' (p. 311)! Never mind that Calvin himself continued to refine this work through 1559-60, and that these final editions of the Institutes (not that of 1541) were the standards that fed subsequent Reformed theology.
When Cottret does speak of Calvin's theology from the Institutes (and elsewhere), he is surprisingly clumsy and extremely condescending. According to Cottret's Calvin, the Old Testament patriarchs have 'a right to salvation' (p. 317). A right to it? Can anyone so grossly misunderstand Calvin's soteriology as to speak of human 'rights' before God? (But perhaps this is just a very poor translation.) In Cottret's estimation, '"election," "faith," "vocation," and "conversion" are practically equivalent' in Calvin's theology (p. 322). Well, that just simplifies everything, doesn't it? Calvin, we are assured, was never fully convinced that the doctrine of the Trinity is exegetically warranted (308), and his disagreement with other Protestants over the nature of the Lord's Supper 'was linguistic before it was theological' (340). Eh? Calvin's commentaries (look no further than that on the Prologue to John's Gospel) are by no means lacking in trinitarian confidence (or did Cottret check these?), and simply because Calvin debates the meaning of words does not make the debate a matter of linguistics. We learn that, in his entire teaching about predestination, Calvin was 'not wise', but was 'carried away by polemics and his authorial vanity'; moreover, he took a 'malign pleasure' in this 'system of death' (p. 322). 'Poor Calvin, a victim of his system' (p. 323)! One may certainly disagree with Calvin's doctrine of predestination, but so to caricature both the doctrine itself and Calvin's intention in teaching it hardly counts as good history.
As a final example of Cottret's carelessness and doctrinal confusion, take his statement on p. 337: 'Calvin's Christ is "at the same time the God who elected and the man who was elected."' Cottret footnotes here Karl Barth's Church Dogmatics II/2, p. 1 (in the French edition-no thanks to the translator). Apparently, Cottret thinks that, because Barth is classed as a Reformed theologian, what Barth says must be what Calvin said. In fact, Barth chastises Calvin on precisely this point, that Calvin saw Christ as the prototype of elected man, but did not see the implications of the fact that Christ is also the electing God (see, for example, pp. 110-11 in the English edition of Barth's Church Dogmatics II/2). If Cottret had perhaps read Barth's preface, he might have caught the following hint: "I would have preferred to follow Calvin's doctrine of predestination much more closely, instead of departing from it so radically" (p. x). So not only does Cottret think he can make statements about Calvin's theology with no reference whatsoever to Calvin himself. He also thinks he can glance over a few pages of a recent work of 'Reformed' theology and assume he's getting pure Calvin. This is inexcusable negligence.