I have been asked to write books too, but I go with a no response always. My reasoning is generally that my I have read free books and learnt a lot of what I have ( was not really rich to buy all trendy books being a middle class Indian student) , my book whenever it is written would be open and free access. That will be my way to pay back. If I have to make money , I will rather do it through my work, not letting publishers have control over the knowledge source I write.