Here is a fair (i.e. evenhanded) review of Walter Borneman's Polk: The Man Who Transformed the Presidency and America, which as I've said before is in my opinion the best of the Polk bios out there. She raises a good point about Polk being a slave owner, a fact that is glossed over in nearly every Polk biography I've read, the exception being William Dusinberre's Slavemaster President: The Double Career of James Polk, which is exclusively about that subject. (And also is rather dull, unfortunately.) Which I suppose is the way Polk wanted it. Drawing attention to his slaves would have done him no favors in a national election, even in 1844.
Another good point: the subtitle notwithstanding, I don't think Polk really changed the Presidency all that much. The President from that era of history who has the best claim on having changed the Presidency is Polk's predecessor, the largely forgotten John Tyler. Back in those days, the Constitution was somewhat vague with regard to Presidential succession--it said that the VP would assume the "powers and duties" of the President should the office become vacant--and some people thought that when William Henry Harrison died, Tyler should only be considered acting President. But Tyler would have none of that; he insisted on taking the Presidential oath of office and even went so far as to refuse to acknowledge documents that referred to him as anything other than President, which set the precedent for all future Presidential successions until the policy was made explicit with the ratification of the 25th Amendment.