Alright, so you're looking for 'The Ritual' by Adam Nevill. I'm assuming that's the one, the horror novel about the guys getting lost in the Scandinavian wilderness? That book legitimately creeped me out for days after finishing it, the atmosphere is just so thick with dread. Regarding reading it online for free, your most straightforward and legal option is going through your local library's digital service. Most libraries use apps like Libby or Hoopla. You just need a library card, which is free to get, and you can borrow the ebook or even the audiobook for a set period. It's not technically 'buying' it, but it's a free, above-board way to read it.
I'd advise against just googling 'read The Ritual free' and clicking the first result. A lot of those sites are packed with invasive ads, might not have the complete text, and are honestly pretty sketchy from a security standpoint. More importantly, it doesn't support the author who created this wonderfully terrifying story. The library route supports authors through lending models, so it's a win-win. The audiobook is also fantastic, if you're into that format – the narrator really nails the mounting panic.