Probably the most authoritative and complete biography of Lincoln, and the one that I think will be the seminal biography for years to come, is
Abraham Lincoln: A Life by Michael Burlingame. It is a 2-volume massive biography that is exceedingly well-written and loaded with information. I bought it when it was first released by Johns Hopkins Press and spent many hours reading and enjoying it.
I bought the
2-volume hardcover but it appears now to be available in paperback, audibook, and ebook formats. The ebook is available from Johns Hopkins (and probably elsewhere) in 2 volumes:
volume 1 and
volume 2. The ebooks (and the pbooks) are not inexpensive, but if you really want to know Lincoln inside and out, this is the book to read.
A Team of Rivals, which was mentioned by jgaiser, is also excellent, but is not a substitute for a complete biography. It is narrowly focused on Lincoln's cabinet and the interaction between men who disliked each other, and even disliked Lincoln in some instances, but how Lincoln and they rose to serve their country and put aside much of the pettiness of politics (but not all). It is worth reading on its own.