I wouldn't send a letter. I would call my dad. If he was not expecting this expense, then your question would catch him off-guard. If you were expected to pay for your books, you would need to have a reason why you don't have the money saved and how you will pay him back for this expense.
I agree this app may be perfect for people who always wanted to have a side income on the internet. If I didn't use https://tabtrader.com/articles/golden-cross-and-death-cross-explained or any trading one in general, I'd definitely try mining Bitcoin, but for now, I don't have a need for that. But who knows, maybe one day I'll be interested in that way of earning money as well.