Andrew Olmsted is right to highly recommend the Jefferson biography American Sphinx:
Andrew Olmsted dot com: American Sphinx: Thomas Jefferson is probably the most popular of the founding fathers. Washington did not leave a great legacy of documents behind him. Adams was viewed as a poor President. Jefferson wrote some of the best-known words in American history and his administration presided over the largest expansion of the republic in its history.
American Sphinx is not a biography of Jefferson. Instead Ellis has attempted to delve into Jefferson's character by focusing on five shorter periods in Jefferson's life. While readers looking for a more detailed review of Jefferson's life may be disappointed, Ellis's study of Jefferson offers a fascinating look into a man whose life is often obscured by the legend that has formed in the years since 1776. While some of the areas Ellis chooses to focus on are obvious and well-known, such as the events surrounding the Declaration of Independence and the first term of his presidency, others are less traveled ground such as his time in Paris and his short-lived first retirement in the 1790s. The combination of the two makes for fascinating reading, particularly as Ellis has a gift for highlighting facts that, while they may be well known to historians, they can still surprise casual students of history.
Ellis's writing style is tailored to read almost in the style of well-written fiction, drawing the reader in and making the book difficult to put down, an all-too-rare trait in nonfiction. It helps that Jefferson led a fascinating life, of course, and that Ellis is focused closely on some of the more interesting aspects of that life. A poor writer, however, can ruin the most interesting of topics, so Ellis deserves credit for writing that is good enough to be almost invisible, letting the reader enjoy the subject without trying to impress with unnecessary verbiage.... [I]f you are already familiar with Jefferson and are looking for something more in-depth, American Sphinx is a fine choice...