Of course you can write books about cryptocurrency and sell them. I see a lot of books about cryptocurrency being sold online. The thing is that you will have to be promoting the books and letting people know about them… if not it might not even sell. Another thing that is very important is the topic you're writing about. If you're like those people that just copy what is already written online then you' will probably be wasting your time and nobody is going to be buying what they can find easily online and even learn it for free. You should try to write about something that they can't find, something that is very important and you know that people will be interested in reading and knowing about it. So that's all I can say. If you check purse.io you will see some crypto related books up for sell.