Heya! I'm not a guinea pig owner myself, but I have friends and acquaintances in the rescue community who work with them primarily. They're fun little critters! If you get the opportunity, finding a guinea pig forum can be very beneficial. If get end up in a last-minute situation where you desperately need advice, they're often the best rescources in a pinch due to members' experience. Pro-rescue forums tend to have the most info, though of course not always.
The guineapigcages.com site is a fantastic resource. For the medical side of things, Guinea Lynx is a very good reference.
http://www.guinealynx.info/
For example, their information on diet:
http://www.guinealynx.info/diet.html