Impeachment by Jon Meacham - Read this one last year and it was likely my favourite read of the year. It lays out the case against each of the three (at the time) Presidents to have been impeached (or in Nixon's case, soon to be Impeached). Absolutely fascinating.

Leadership in Turbulent Times by Doris Kearns Goodwin - This one is more personal. It focuses on three Presidents, their careers, how they came to be where they were and how their 'biggest' moments came to define them.

That's all I've read so far, although I've only been 'interested' in this as a subject for about three years. Hope it helps.