Looking for the official Tokyo Ghoul light novels online is a bit of a project. From what I've gathered, there isn't a single official, legal English ebook release for the light novels like 'Tokyo Ghoul: Past' or 'Tokyo Ghoul: Void' that you can just buy from Kindle or Kobo. The main manga series is widely available digitally, but the prose side stories seem stuck in a weird licensing limbo. Your most legitimate bet might be to hunt for physical copies of the English translations by Viz Media, which are often sold through major online book retailers.
That said, I've seen fan translations floating around on various manga aggregate sites and forums for years. The quality varies wildly, and obviously it's not supporting the creators, but that's how a lot of people, myself included, first read them. It's frustrating because the novels add such interesting context, like what Kaneki's college life was like before everything went wrong, or how Touka ran Anteiku. You'd think with the anime's popularity, they'd make the digital versions more accessible. I ended up ordering the physical books after reading fan scans, just to have them on my shelf properly.
If you're dead set on reading them online legally, keep an eye on Viz's digital storefronts or services like Shonen Jump's app. Sometimes they add older ancillary material slowly. Until then, it's either the physical route or navigating the less-official corners of the internet, which I can't really recommend but I know is common practice.