In study session 4 (page 318), the interest rate parity formula is: F/S = (1+Ra)/(1+Rb). On the next page there is an example to calculate a 3 month forward contract when given annualized interest rates, making each interest rate (1 + R/4). In Derivatives, the formula is F = S x (1+Ra)^T/(1+Rb)^T. (page 27, 49) Why is it different and which one is right? Does it have to do with annualized rates?

Yes, when the problem gives you annualized rates, you need to adjust the formula to reflect that. The first formula above is assuming that you are calculating a forward rate in 1 year.