This is not a bad deal: thirty-eight Discworld novels for eighteen bucks. I mean, yeah, I have them all already, and you can tell the precise moment in my life where I could afford to buy Terry Pratchett in hardcover. Still, it’s a damned good Humble Bundle offer.
One caveat, though: you’ll need a Kobo account. I have no idea how difficult it’d be to get those books out of there, and into your Kindle reader. …And I suppose that the fact that I have to make that caveat tells you quite a bit about Kobo that the platform would prefer you not to dwell upon.
My experience with Kobo a few years ago was that they were very effective at tying you to their platform with chains of metaphorical steel.
Kindle has gone from mobi to epub as default, so it shouldn’t be a problem.
But you’ll still have to sideload or use that “email to yourself” option.