I have been teaching my young grandchildren Scratch on an RPi for a few weeks and found the following books useful.
First, the MagPi Essentials book "Learn to Code with Scratch". Elementary, which is what we need. Being about Scratch specifically on the RPi, it introduces the GPIO.
Secondly, "Scratch for Kids" for Dummies. However, it is not machine-specific, so does not cover GPIO.
Lastly, "My first Computer Coding Book" by Usborne. Unfortunately, it focuses on something called Scratch Jr, which I don't think is available for the RPI - but I think is used in schools.
Hope that helps,
IT Background: Honeywell H2000 ... CA Naked Mini ... Sinclair QL ... WinTel ... Linux ... Raspberry Pi.