I want to get a book once AND ONLY ONCE??

You need to move the ClickDetector connection to be outside the hit function, otherwise it will never get a chance to run. You also don't need the inner hit function. Something like this should work:

local var2 = 2

function hit(x)
    local y = x.Backpack
    local z = game.Workspace["TextBook"]

    if var2 == 2 then
        print(var2)
        z:Clone().Parent = y
        wait(3)
        var2 = 5
    end
end

script.Parent.ClickDetector.MouseClick:connect(hit)

It looks like you're creating two hit functions, so the first one is being overwritten. Be sure to use different (and preferably somewhat descriptive) variable and function names while programming. The reason why it's not doing anything, however, is because you've enclosed all the code inside the first hit function (be sure to indent please).

My suggestion would be to rewrite your code entirely because you forgot to put end to end statements, making it impossible for me to determine how your code was actually intended to be formatted. Also again, push that tab key.

There is some problems on the base I have noticed. You should say "local" before a variable name and you should NEVER add a number in a variable, it breaks the variable. You also named the functions the same thing, so the game doesn't know which one you mean.

local variable = 2
function hit(plr)
  local  y = plr.Backpack
  local  z = game.Workspace["TextBook"]
 end
function push(plr)
    if variable == 2 then
    print(variable)
    z:Clone().Parent = y
    wait(3)
    variable = 5
end

script.Parent.ClickDetector.MouseClick:connect(hit,push)