So the first 3 weren't originally published in hardback, that came after the later success of the series and only came as part of a 3-in-1 box set (you can see the box around OP's first 3 books in his photo).

There's not yet been another bundled hardback collection, so if you want to buy an actual set that has a nice box that holds all 9, you're gonna have to wait it out. Otherwise, you can buy the first 3 box set and the individuals of 4-9, or you can order this $250 bundle on Amazon, but it's just a seller's bundle and not a unique official box set so it might not even have the box for the first 3 books.