I have the book print version and that certainly is outdated. But the online version isn't outdated (mostly). The current version of Yesod[0] is 1.4 and according to the book site it targets 1.4. That being said contributing to any of Michael Snoyman's (main yesod developer) project (including the book) is easy and fun. He is very responsive and friendly. :)

[0]https://hackage.haskell.org/package/yesod