Comme:
Hi to everyone!This is my first post and i hope is going to be helpful.
I found somewhere in the internet another way to connect the piezo. It works, and the values returned (in my case) are between 1 and 10. I attached the scheme of the connections.
Furthermore, the code in the book when the counter goes to three is wrong: it does not reset the counter numberOfKnocks, causing a persistent state of "unlocked".Here is the code:
if(numberOfKnocks >= 3){
locked = false;
myServo.write(0);
delay(20);
digitalWrite(greenLed, HIGH);
digitalWrite(redLed, LOW);
Serial.println("The box is unlocked!");
numberOfKnocks = 0; //line added
}
Thank you Comme! That helped me so much. I was having such a hard time figuring out the wiring. Now it's working, and I can finally go to bed! 
Thanks again!