Zelda Lore shared an excerpt of the book on Twitter (thanks to Gaming Reinvented for the tip) which covers the potential whereabouts of Kass in Tears of the Kingdom. The bird isn't present in Hyrule at all in the game, with no explanation at all given for his absence. He's not even mentioned. At least, until the Master Works book.
Before we jump in, we want to stress that these aren't canon; as we said in our review of the book, it "doesn't spoonfeed answers" and that the "History section is eager to give details on what you already know, but only poses questions for some of the juicier secrets". Now, with that out of the way, potential spoilers ahead (after the Smash Ultimate version of Kass' Theme)
Zelda Lore gives us a rough translation of the section on Kass — essentially, the book suggests that Kass may have either left Hyrule to learn more, or he's flying around Hyrule and somehow we happen to never run into him. Both suggestions are very vague, so don't take either of these with any
Interestingly, Penn, the reporter in Tears of the Kingdom, is mentioned in this entry: he may have "heard about [Kass] somewhere, or perhaps he is feeling the same way as the unknown poet".
So it sounds like we'll never officially know where Kass has gone. We'd love to see our favorite Rito again, as he was such a big part of Breath of the Wild. Maybe one day Kass will return, but for now, plot convenience seems like the easiest reason.
Ah well. Good thing the Master Works book is rather lovely, even though it may not answer many questions. Still, it's a must-buy for Zelda fans, and hopefully an English release will follow soon.
Let us know what you think of these suggestions in the comments below.