Here is Apple's iOS App Support page.
Apple - Support - iOS Apps
Choosing Numbers and then Getting Started with Numbers will get you here.
http://help.apple.com/numbers/ipad/1.7/
This is basically the same information you can find by selecting the wrench icon and Help in the app.
I have an older book for the iWorks apps, but to be honest it wasn't as clear as Apple's help pages, and didn't cover a whole lot more. At least not the stuff I wanted to know more about. Like you, that was mostly about how to enter and use formulas.
The only other recommendation I have is to explore. Make a scratch spreadsheet, then explore how to enter formulas. Especially work your way though the various Functions and their descriptions. You don't have to understand how to use them all, but it will give you an idea of what is in the tool box. Then you can concentrate on understanding those that you do want to use.