Well any discussion of works on Lee has to begin with Douglas Southall Freedman's epic work Lee. Freeman's work, in four volumes, was finished in the mid 1930's and won a pulitzer prize. That being said, Freeman was hardly an objective author and he deified Lee while helping to advance the Lost Cause ideology. So, as long as you are aware of the book's slant it is still worth the read because of its sheer detail and groundbreaking nature.

I've heard good things about Michael Korda's Clouds of Glory, but unfortunately I haven't had a chance to read it yet so I can't say anything definitive about it.

Another one that I have read though is Charles Bracelen Flood's Lee: The Last Years which is focused solely on Lee's time at Washington College (now Washington & Lee University) following the war until his death in 1870. What Lee was able to accomplish in Lexington in five short years is incredible. Reading Flood's work truly made me admire Lee, but, full disclosure, Flood makes no qualms about the fact that he admires Lee as well, so his work can be a little apologetic and one-sided as well.

Hope this helps!