Well, not as helpful as an instruction book, but:
When you select a cell and go into data entry mode you'll see an equal sign in the tool bar. If you select that you go into formula entry mode. Choose functions form the keyboard and you will see a list of Recent functions, or choose Category to drill down through all the formulas.
Now here is the part that might be of help to you. If you tap the blue arrow next to the function it will give you a brief description of the function and it's syntax.
Also, if you tap the wrench and choose help from the menu it will take you to Apple's support site for iWorks. Under Numbers Help you will find a header for "Calculate values using data in table cells." This gives a fairly decent overview of how to enter and edit formulas.
Hope that helps, at least a bit.