If you're going to recommend a web site instead of a book, I would much rather point him here:
"Evolution 101": http://evolution.berkeley.edu/evolibrary/article/evo_01
I just want to say that I would not recommend Dawkins's The Greatest Show on Earth. I'm a strong atheist, a fan of Dawkins, and I greatly enjoyed that book, but it is rather harsh on the perspective of believers. If you're looking to convince someone of evolution, anti-theistic rhetoric will not get the job done. Instead, I'd suggest Coyne's Why Evolution is True, Shubin's Your Inner Fish or even Dawkins's The Selfish Gene.
Mendel did write to him, but Darwin apparently never read it.
While I love Dawkins' work and would highly recommend "The God Delusion" and "Climbing Mount Improbable" I'm going to recommend a non science book.
"Misquoting Jesus"
First it's written by a Christian and should be a little less scary then Dawkins to other Christians.
Second it mainly focuses on how the English translation of the bible cannot nor should not be taken literally.
The author was a born again Christian and the more and more he studied the bible by learning Latin and ancient Greek the more he came to realize that the English translations that most people are always quoting is riddled with errors.
I think that if you can at least ease a creationist into the idea that the bible should not be taken literally that is at least a good start.
As I understand it, Mendel wrote Darwin a letter about his theories, but Darwin never read it.That's a myth, if I recall it right. It stems from the false rumour that Darwin owned a copy of the German journal in which Mendel published his work. It's not even a sensible myth, given that the history of evolution would have been radically different from what it is if Darwin were to have a genetical understanding of his theory.
I thought maybe this short video could be useful to show to a creationist:
It's good old Sagan, no bashing of religion a la Dawkins and Coyne. But I think it could be considered offensive to a religious theist because Sagan dismisses the idea of design, on which theism rests. I'm starting to think this is unavoidable. The creationists rail against evolution precisely because it is so corrosive to the design hypothesis.
I really didn't come here intending to shoot down everyone else's ideas, but I'm not sure I would recommend Victorian prose written by the Father of All Lies himself to a home-schooled creationist, either.
As I understand it, Mendel wrote Darwin a letter about his theories, but Darwin never read it.
Do you have a source for this?
What a pity therefore that Darwin never got his hands on the paper and read it or that Mendel never wrote to Darwin and explained what he was up to.
That's a hard one to read if you have no understanding of biology though =P I bought that for my mother for the same reason but she couldn't really read it, even if she was receptive to it.
I recommend "The Bible Unearthed"