Today, 1:08 pm
I happen to have the book in question by Roland Bainton. There is nothing in Bainton's essay "The Bible and the Reformation" [Studies on the Reformation (Boston: Beacon Press, 1963)] that suggests "he believes that Luther might have changed the canon if he thought he could get away with it," and for that matter, "Bainton, who normally gives Luther every break possible," actually Bainton presents a short essay that includes critical reflection.
The actual comment on Luther and the canon is more of a passing comment that was part of Bainton's broad overview on who had the authority to interpret the Bible during the Reformation period. Bainton points out that "Luther epitomized two centuries of antipapal critique" (p.4); that is, Luther wasn't doing anything new when he questioned whether or not the pope had the infallible ability to interpret the Bible, this debate had been going on for quite some time. Bainton presents the argument given back to the Reformers that the church created the canon, therefore she had such infallible authority. Bainton says the Reformers countered that the Gospel or "Word" made the church, so the Gospel or "Word" is above the church, and therefore the church must be submit to the authority of the Gospel or "Word." Bainton cites "a canon lawyer at the Council of Basel" who had earlier reflected Luther's view "in matters touching the faith, the word of a single person is to be preferred to that of a pope, if that person is moved by sounder arguments from the Old Testament and the New Testament" (p.4). The point is that this issue of Biblical interpretative authority was nothing new when Luther showed up on the radar.
As JonNC has explained the canon was also a related issue during this time period. Luther's solution was (in part) to evaluate the canon by the "Word." When Luther did so, certain books accepted by broad Tradition appeared to lack a pedigree of containing the "Word," but as Bainton points out, "Tradition at this point was presumably too strong for him." In essence, one sees that Luther was being cautious (for instance, simply compare Luther's early preface to Revelation with the later revision).
The heart of the issue is that Luther questioned the infallibility of the church, and this questioning included whether or not the church infallibly determined the canon, linked with this was the confusion present during the Reformation period. M. Reu notes,