About English plural 's'

'Five books' is a piece of idea. From the word 'five' we know it means more than one book. The question is why we put an extra 's' to denote it is a plural? If we don't put this 's', shall somebody mistake what we mean to say? This is another example of double expressing. Of course the following question would be why and when shall we double express and why and when we express the same idea one time or three times. For five books, it is easy to get an integer number. Yet, if we want to express something in fraction, it would be a nuisance. 1.23 pies comparing 0.23 pie, are they two different things? Something like 1.0000001 dollars, we put 's', while 0.99999 dollar we don't. Beside this, we have to know the different among 1.0001 percent, 0.999 percent, 101 percent, 99 percent etc. There is no logical reason but just rote. Sometimes, a number we can't see it directly such as log7.8+sin46/tg7=? Voltage(s). In other case, a question itself is about whether a certain value is greater than one or not. If a teacher ask students, that A/B apple(s) is (are) greater than one apple or not, it would be very hard for the use to put 's' or not. For either put or not will tell the answer too. As for the negative number, there is some thing more to learn. In positive number we know that the number greater than one, we put 's' as plural, while for the negative number, we put 's' after the number less than -1 and when it greater than -1 we won't. That means to say, when the absolute value smaller than one, we don't put 's' and when the absolute value greater than one we put. Do you think we can explain this to a toddler? If not then, what we tell them is an unstable knowledge and no one can remember an unstable knowledge as well as a stable knowledge. All these troubles are from the ancient usage of 's'.