Many people hold President Lincoln as the greatest American president for holding the states together during a period of civil war and for freeing the slaves. But recently I have found a letter where President Lincoln said that if freeing the slaves would keep the states together he would do it, if only freeing some would keep it together he would do it, and if freeing none of them would keep it together then he would do that.
While paraphrased a bit it’s basically saying that he would do what ever he could to keep the states together. So was his freeing of the slaves just a bid to try and keep the nation together or did he actually care for the welfare of the people he was freeing?
There are some quick online resources that you can use to get started:
This brief study of Lincoln's writings on slavery contains examples of Lincoln's views on slavery. It also shows one of his greatest strengths: his ability to change as it relates to his public stance on slavery.
See also the history of the 13th amendment as portrayed in the recent film "Lincoln"
The important thing was that he could change his mind and grow as a person. This is an important concept that some people miss.
(Today this would sometimes be called "flip-flopping" on a issue.)
The Fiery Trial by Eric Foner is an excellent place to start. According to Foner (I'm not sure if it's included in the book but I heard him say it in person) that letter was an open letter to Horace Greeley to be printed in the newspaper and it was written when Lincoln had a preliminary copy of the Emancipation Proclamation in his desk. Lincoln was preparing the American public to accept that emancipation was crucial to the war effort. While Lincoln did not that think that African Americans were equal to whites he believed that they should have access to have many of the same inalienable and basic rights
This may diverge from your interest in Lincoln himself a bit, but here is an interesting article from an international relations perspective on the relationship between the slavery issue and the looming issue of British involvement.