As of today, you can get Nightmare City either as an ebook or paperback novel on Amazon. If you preordered it, it should be delivered to your Kindle automatically today.
Otherwise, buy the Kindle version for Amazon.com here, and for Amazon.de here. You should also be able to access the print versions from those links, if reading a proper book is more your speed.
Also, I’d be eternally grateful if you wrote me an honest (!) review on the platform of your choosing (Amazon, Goodreads, Facebook, whatever). Reviews are super important for the success of a book, and I’d really appreciate it. Thank you!