Lincoln was an abolitionist, a quite determined one. But he was above all else a lawyer.
The Constitution did not permit the abolition of slavery. His commitment to the law demanded that abolition of slavery wait until there was legal authority to do so. That meant ending Southern control of the national government, which meant restricting the expansion of slavery. It was his adamantine commitment to that which led to the Southern revolt.
The author of the book incorporates bits of real historical documents in a coherent fashion. Somebody said a hack is someone who does well what shouldn't be done at all. It's moronic tripe like this that makes that definition seem true.