It was also the first non-Oz book that I ever came across by Baum. It wasn't exactly a surprise that he had written other things but it was important for me to find that out. For what it's worth, that also means that I first read the Master Key more than 20 years before I read a lot of his other books.
Here goes a quick summary with plenty of spoilers:
A boy named Rob, who is absolutely crazy about electricity and electronics, accidentally makes the right combination of wires and connections to touch the Master Key of Electricity and summons the Demon of Electricity, although the genii of electricity is probably a better description.
The demon, who is one arrogant being and totally dismissive of humanity, decides to give Rob three devices a week for three weeks in order to spread the wonder a knowledge of electricity.
In the first week, Rob is given a bracelet that lets him fly at transatlantic speeds, a wireless taser and pills that replace food. He goes off on some fairly reckless adventures, getting lost and barely making it home.
On the second week, Rob gets a garment that is basically a force field, a screen that will show events around the world and a pair of glasses that will reveal someone's character. He shows more foresight than he did last time, although he still ends up globetrotting and even helping out some foreign governments.
Come the third week, Rob has had enough. He's come to the conclusion that all these devices are alienating him from his family and that people really are ready for this kind of stuff yet. He rejects the future gifts of the demon and gives the other gifts back. The book ends with a depressed demon hoping to someday humanity will be ready for him and Rob happy to be done with the whole mess.
The Master Key has probably aged more poorly than a lot of Baum's books. While I will agree that things like computers and TV sure seem like magic, the inventions are just too fantastical. In particular, the glasses that basically amount to limited telepathy is straight up magic.
On the other hand, Rob does show more character development than most of Baum's heroes. He goes from a reckless idiot to a mature young man. The book is more about his growth as a person than his adventures with the demon's devices. He's still pretty flat as a character but he does mature.
The Master Key is a cross between an Edison fantasy and a tale from the Arabian Nights, making it unique if noting else.