I have been reading a lot of reference books about the U.S. Presidents lately and have found one I particularly like. It's Our Country's Presidents, written by Ann Bausum and published by National Geographic Society. While it's chock full of information, it's designed to be easy to use and includes a great many interesting illustrations. I recommend the fourth edition, published in 2013, for ages 10 through teens, and think adults will also find it interesting.
(Cover art courtesy of National Geographic Society)