User blog comment:LittlePeng9/How many books are there?/@comment-5529393-20130430044350/@comment-1605058-20130430051655

I figured out of what order this number is. It's pretty obvious that if n<m then b_n<b_m. So for every n we have b_n<=b_M. This can be 10^4900 pages at most, so summing gives Sum_i {b_i} < b_M*10^4900~64^10^4900*10^4900, so this upper bound also shows that numbers are comparable. Your bound is tighter, of course, but my one also makes a point.