Only once per account, I'm afraid - mainly to prevent someone from gaming the system to turn $70 into $500 worth of stuff. But you will move up one reward tier above whatever donation you make, so $10 becomes $25, but, at the same time $250 becomes $500, etc. All additional referrals will go towards your shot at the Mythic Camper Award, though, so it never hurts to spread the word a little further :)
All money raised will go directly back to the world of Camp Myth, and ensure the release of future titles in the series. New cover art, illustrations, distribution, promotion, etc. With this campaign, you're not just backing one book, but the potential viability of Camp Myth as a whole.
10 and up sounds about right. I want this series to be accessible for kids, while at the same time be something I would read myself. Kind of like how an entire family can watch a Pixar movie and each person can enjoy it for different reasons.