This is the closest we may ever get to a definitive biography of Thomas Jefferson. Written by one of the greatest Jefferson scholars of all time, this six-volume, Pulitzer prize-winning monument of a book took Malone the better part of his adult life to finish. It's a beautiful read too. Probably not the biography to read straight through, but if you're writing a paper, and you need to know something in detail about Jefferson's life, Malone is your go-to guy. Your town's library probably has a copy.
Generally acknowledged to be the best one-volume biography of Thomas Jefferson. Peterson, the Thomas Jefferson Foundation professor of history, emeritus at UVA, is probably the greatest living scholar of Thomas Jefferson. His book is the product of a lifetime of study. It's another whopper though, clocking in at just over 1100 pages.
A short, clear, readable, and reasonably complete one-volume biography by a respected constitutional historian. If you're going to read one biography of Thomas Jefferson, this is the biography to read.
American Sphinx is a wonderful and strange book. Instead of trying to tell the story of Jefferson's life, American Sphinx tries to figure out what Jefferson was thinking and feeling. Its overwhelming problem is that of understanding Jefferson. As a result, it doesn't cover his whole life in much detail, but zooms in on a handful of events and uses them to pick apart Jefferson's psyche. It won the National Book Award, which is a pretty big deal. Really readable, and comparatively short too. If you're only going to read one biography, read Bernstein or Peterson's book. But if you're going to read two, you should consider checking this one out.
Scholars have point out that from the time of their first meeting in the Virginia legislature until Jefferson's death in 1826, Jefferson and Madison were so close it was often difficult to figure out where one's thought started and the other's ended. Jefferson dreamed of getting Madison to move near his house at Monticello; Madison succeeded Jefferson as Secretary of State, President of the United States, and even Rector of the University of Virginia. Koch's study of the pair's collaboration, though old, is still a standard work.
This book, an edited collection of papers delivered at a conference about Thomas Jefferson, marked the point Jefferson scholarship took the tack it's still on. If you want to get a sense for the field—the questions scholars are asking about Jefferson, and why they're asking them—this book is a great place to start.