I've never doubted his seriousness, just his research. He has an agenda and he picks and chooses quotes which appear to support it.
The book I can't find is a collection of Lincoln's speeches and writings, which bears out Bennett's thesis.
Lincoln's speeches and writings are online Here.
My recollection is that Lincoln figured the Blacks to be eager to leave the USA.
I think your recollection is wrong. Lincoln was a supporter of voluntary colonization, something many people in the U.S. supported. One supporter who put his money where his mouth was was Robert E. Lee, who paid passage to Liberia for several of his slaves in the 1850's. If you read Lincoln's speeches as writings on the topic in context, you see that Lincoln probably thought that emigration gave blacks the greatest opportunity for achieving the goals of the Declaration of Independence than remaining in the United States did. We were, after all, a country that did not treat blacks as equals, did not afford them the same rights as whites, did not allow them the same freedom as whites, and where many people, including the Chief Justice of the Suprmeme Court, felt that they were not even eligible to be considered citizens.
You asked me to provide some evidence for my statement, and I have. That you care to reject it out of hand is no concern of mine.
With all due respect you have not. Not one of your posts has included a quote from Lincoln saying where he supported the forced deportation of blacks.