To be honest I think better than any specific business resource out there would be to just get her on the path to building the core skills she'll use later on in life. She's already got that started with her vending business which is great. I think pushing her towards taking on some school activities may be great too. Things where she can help organize events and fundraisers and really contribute to a group.
As for my favorite books on business I would suggest for a 13 year old:
"Death by Meeting" The author describes a fictional company that is terrible at conducting meetings until the main character comes and helps address the issues. I know she's not really running meetings at the moment so this may be of less use to her until she has others working along side her. http://www.amazon.com/gp/product/0787968056
I also agree that "How to Win Friends and Influence People" is an excellent book. I grew up with social anxiety disorder and it was one of the books that really broke me out of my shell and gave me the confidence to really push past it. http://www.amazon.com/gp/product/0671723650
Another book I found pretty interesting was "Win the Crowd." While not a 'business' book, per se, it goes into how to really grab your audience's attention and hold it. http://www.amazon.com/gp/product/0060742054
Lastly "Secrets of Powers Negotiating" is an interesting book I'd suggest she take a look at. There are endless applications to negotiation in business from sales to even negotiating your first salary. http://www.amazon.com/Secrets-Power-Negotiating-15th-Anniver...
Just some ideas -- hope it helps!
