We're all show-only now. Only the first season was an adaptation of the book, season 3 will be an adaptation of the unwritten sequel to the first book Gaiman and Pratchett never had the chance to finish before Pratchett passed.
Season 2 was also entirely original straight from Gaiman. He said he felt there needed to be a bridge between the published book and the sequel they planned, so he wrote a story on his own to do that, and that's Season 2.
Season 2 really felt like a stopgap measure to pad out the series, didn't it? I appreciated the character bits but I'm hoping the final season brings more substantial story to the table.