I know that sounds trite but having devoured both of his own texts, any book that carries the stamp of his approval (aside from the Bible, which I've already read) is, for me at least, a must read.
It is a weighty historical tome that attempts to bring light to the rise of Lincoln, his brief time in power and flesh out those that were his rivals and in time, became allies. I'm about half way through it's 900+ pages and it is a marvellous and enlightening book that I cannot recommend enough, both as a biography of greatness but also as a self-help guide to good leadership.
I'll leave you with one of my favourite bits so far, that still speaks volumes today in our divisive times...
Lincoln had nothing but disdain for the discriminatory beliefs of the Know Nothings. "How can any one who abhors the oppression of Negroes, be in favor of degrading classes of white people? Our progress in degeneracy appears to me to be pretty rapid. As a nation, we began by declaring that 'all men are created equal.' We now practically read it 'all men are created equal, except Negroes.' When the Know-Nothings get control, it will read 'all men are created equal, except Negroes, and foreigners, and catholics.' When it comes to this I should prefer emigrating to some country where they make no pretence of loving liberty-to Russia, for instance."