“A Muslim friend of mine rejects the incarnation. He says the following verse makes it impossible: 'God is not human, that he should lie, not a human being, that he should change his mind. Does he speak and then not act? Does he promise and not fulfill?' (Numbers 23:19) He claims that this passage proves that Jesus cannot be the Son of God. How am I to reply to this. I do believe he is taking the passage out of context. Also, what books do you recommend for my Muslim friend?"
You can agree with him more than he may realize, as no orthodox Christian would say that God is a human being. Of course God is not a human being. He is infinitely beyond our mortal abilities, weaknesses, and limitations. Yet the point of the passage is that he is not subject to human weakness. Passages must be understood in context, and it's to scour the scriptures searching for a sentence or two that verbally seem to contradict (or support) our beliefs.
As Christians, we would say this: God took on human flesh in the person of Jesus Christ. Even the Qur'an refers to Jesus as the Word of God. That word, that message, came to us in a better way to understand than words; it came as a human, with whom we can relate (John 1:14,18; 14:6-9; Col 2:9). Whereas Muslims claim that the Qur'an is Allah's word, we have experienced that Jesus is God's word. The scriptures bear further testimony, and it is not wrong to describe the Bible as the word of God.
Further, at the time Numbers was written, it was literally true that God had never been a man. But over a millennium later, the incarnation changed everything! Yet relating this truth requires cultural and spiritual sensitivity. That's because in Islam, the most serious sin possible is associating others with Allah -- for example, claiming that he has a wife or a son, or rules with another deity. This unforgivable sin is called shirk. The central teaching of Christianity (the incarnation) is the unforgivable sin of Islam!
As for material for recommended study, I have lots of material at my website to help you and your friends with your questions. There are 5 podcasts on Islam (Podcasts > World Religions). And my 2009 book Jesus & Islam should also be helpful, especially in shaping the right attitude towards Muslims (the attitude of Christ).
A caveat: I probably wouldn't hand a Muslim a book denying of the claims of Muhammad. A better use of the material is for your understanding. You read and listen and study, then share whatever your friend needs to hear. When we're trying to explain ourselves in an atmosphere of tension or disagreement, face-to-face is nearly always better than an email, or even a phone call. The observation holds doubly true when what we're sharing what might be construed as heretical or dismissive of another's faith (or insulting -- and as we're frequently reminded in the news, any statement that could be interpreted as dismissing the claims of Muhammad, or elevating another religion over Islam, is likely to be provoke a strong negative reaction).