In this question, Greg considered the possible arrangement being Novel, Biography, Novel, Biography…
However, can we also not arrange them as Biography, Novel, Biography, Novel… → In this case, wouldn’t the answer be 2(n!)(n+1)! ? I had initially solved this algebraically and reached 2(n!)(n+1)! myself too…
Consider the case where, for instance, n = 2. The total arrangements of N_1B_1N_2B_2N_3 is n! \cdot (n + 1)! = (n!)^2 (n + 1). Your logic of multiplying by 2 assumes that you can also have an arrangement like B_1 N_1 B_2 N_2 B_3 as your symmetric “other half”, but that contradicts the “n + 1 novels and n biographies” part of the question.
