Everybody misunderestimated him. Seward even sent him a memo intimating Lincoln should delegate power to run the government to him, because Lincoln couldn't handle it.
If they had read any of his speeches prior to that time, they would realize this was no average mind. This man was exceedingly Brilliant. He could write with an eloquence that I have seldom seen matched elsewhere in History, let alone among his contemporaries.
And now is the time to mention my theory about "Brilliant Presidents." They screw things up utterly. If you look back at the history of our Presidents, I dare say you will notice that all of them who were considered "Geniuses" were the ones that made the biggest mess of things, Obama and Clinton being but the latest examples.
Except for the fairly obvious fact that when he took office he was, as he said, faced with a task greater than that faced by Washington. He did not cause that mess, he simply had to deal with it.
IMO no other American president, not even Washington, would have been capable of pulling the country intact, however damaged and diverted from its original course, through the mess.
I have a three-volume set of books that contains most of his known letters, speeches, articles, etc.
The personality really comes through, especially in the early private letters that he had no reason at all to believe would some day be perused by scholars.