Process-wise, the first step is to have a solid outline and target audience in mind. That will inform the book's style and keep you on track as you put the proverbial pen to paper. When working on an individual chapter, I like to write most of it longhand. That forces me to rewrite it as I transcribe it to the computer and I find the end-result requires many fewer iterations to complete when I do that. Aside from all that, it's all the same things any author will deal with, things like writer's block, etc. Just make sure you are prepared to really focus on getting it done once you decide to write it ... because there's nothing worse than starting a project like this, and then letting it languish. It will always be on the back of your mind and you won't be able to enjoy relaxation time until you get back to it.
Anyways, hope that helped, happy to answer any other questions you might have :)
Great news on the book, we definitely need to start seeing more books out there on Mono/Xamarin tech. Hopefully we can help people understand what a great opportunity C# can be when you embrace it :)