Ward Lamon was Lincoln's friend, his book is a good insight into Lincoln as a person, and has some good witticisms too.
I have attached the copy from the Illinois Digital Archives here.
There's also a book called Wit and Wisdom of Abraham Lincoln, I'm sure in a library, somewhere, I think we got one from the house in Springfield.
Billy Herndon's biography also includes some of Lincoln's wit. I haven't read it myself, but it's mentioned in Mr. Tripp's book(The Intimate World of Abraham Lincoln). Mr. Tripp mentioned some of Lincoln's off color stories, though only a few of them were recorded. You won't find them in Herndon's or Lamon's works.