There's been so much written about Lincoln this year it is no surprise that there is more attention paid to his birth date than (perhaps) normal. OUP has a series of three articles beginning with this one on the President where they interview Allen Guelzo, author of Lincoln: A Very Short Introduction.
Often cited is the fact that Lincoln is the most written about historical figure. Well with little to interest myself last weekend I took a look at BIP via the New York Public Library. According to the database (and this is an unscientific query), there are 1,577 active titles, 25 forthcoming and 519 inactive nonfiction hard or soft cover books on the President. That's a lot especially compared with some other notables such as Washington (1480 active), WSChurchill (521), Napoleon (1143) and Hitler (805). I checked "God" just in case and he/she has 8,312 although I am not sure how those are classified.
OCLC's identities also shows an interesting perspective on books about and by Lincoln. Here. They show fully 18,714 works in 26,693 publications in 66 languages and 1,036,286 library holding. The identities shows how constant has been the flow of books about Lincoln over the past 100 years. It looks like there is always between 350 - 450 new Lincoln books per year.