Yes, if you wanted, you could always buy invisible ink that appears when you shine a blacklight on it. But your friend would have to get one, too so they could read it. But if you're not into spending money or, you're just plain broke, there's a cheaper way that works just as well. So, are you ready for the possibly life changing secret invisible ink formula?
It is... Lemon Juice
That's all. You could add vinegar, too, but, it would smell bad. Just paint with it on the paper and it will appear invisible until it is held over a candle. Now, I know you're not a five year old (if so, that's awesome) and this is already pretty obvious but, please, don't catch the paper on fire. If you can't use a candle you can also use a hairdryer. A hairdrier would not be so useful if you were trying to be quiet though. Spying is a dangerous business.