May I also know the book reference so that I could follow.
We the human is also a sign of Allah because we are ashraful maqluqath created by Allah. And in humans we the Shia and momin are the best. So can every momin call each other Ayatullah Al uzma? Or is it only for those who are Islamic scholars? If yes may I please know the book reference?

These all discussion are based on linguistic field for finding the meanings of Ayat or Ayat al Uzma no need to refer any hadith, just you refer any Arabic dictionary you can find the meaning.
Now as far as the word Ayatullah al Uzma is concerned. There are many people they have been brain washed by some mischief mongers that why should we call our Marja as “ Ayatullah al Uzma”. If it’s meaning is “Signs of Allah” therefore all the creature and creation of Allah are the signs of Allah but why don’t we call all of them “Ayatullah”. Answer is no problem you can call but if this question is because why we have to call them only? Then I would say One should first know about the merits of an Marja then he will understand well why we are calling them Ayatullah Al Uzma.