: The official source for the most recent chapters. You can typically read the first three and last three chapters for free, with the entire series available for one-time reading via their app.
: While there isn't a single "paper" book containing the entire run due to its length (currently ongoing at 155+ chapters), you can purchase complete sets from retailers like eBay or local bookstores. Recommended Reading Order owari-no-seraph-manga-read
Are you planning to start from the of the story, or are you looking to pick up exactly where the anime ended ? : The official source for the most recent chapters