User blog comment:LittlePeng9/How many books are there?/@comment-5529393-20130430044350

The number of possible books with an arbitrary number of pages is about the same as the number of possible books with the maximum number of pages. If we let M be the maximum number of pages, and b_i be the number of possible books with i pages, we have b_M = b_{M-1} * 64^{100}, and b_{M-1} = b_{M-2} * 64^{100}, and in general b_{i+1} >= b_i * 64^100. So

Sum_i {b_i} <= b_M (1 + 1/64^{100} + 1/64^{200} + ... = b_M (1 / (1 - 1/64^{100}) ) ~ b_M.