Is the exponent bias $2^{n-1}-1$ or $2^{n-1}$
Both encodings for the exponent are possible, and they are called Excess-$2^{n-1}$ and Excess-$2^{n-1}-1$ - in particular, you might meet such number encodings as Excess-127, Excess-128, Excess-1023 and so on.

The IEEE-754 standard happened to choose the Excess-127 and the Excess-1023 encodings for exponents, so these encodings became more customary. Your book is not mistaken about the exponent bias, it just follows a way, different from this standard.

You can find additional information and nice graphs here.