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.
Writing to earn money is not easy. Writing books to earn money is even harder. Sometimes I come across a useful book full of learning resources and then I see the top 5 books that earn authors the most money and the difference is so wide. Sometimes you have to tap into the society and their mindset to learn what they would like to read. Sometimes it does not make logical sense.
For me in crypto now, that mindset is all about making money!
writing to earn is easy but writing an article or long essay is the hard one as well as writing contents for a book because you need to open your imagination and you cannot just read online and rewrite what you read because that so inapropriate thing to do . you can either get caught for copyrighting or for plagiarising . crypto and blockchain is now popular and creating a books inspired by them would be a good idea as a business because the market for it are not really that huge .