As historical writing is always in conversation with preceding work, I think it's quite useful to start with an older book. In the case of FDR, I think you'd do well to read Leuchtenberg's Franklin D. Roosevelt and the New Deal, published in the early 60s, IIRC. Like all work, it's a product of its time, so some of the claims may not be quite as relevant as they were ~50 years ago.