After fishing forever, I have seen many many Frostwalker II books, but have never seen Frostwalker I. I have never found either in chests, but the world is not greatly explored. Admittedly, this could be due to chance, but I have acquired around 9 Frostwalker II books and never a Frostwalker I book, so I am starting to think this may be a bug with the fishing loot tables.
Update: my statistic for fishing is > 17,000.
- Run this loot command in a repeating command block for several minutes.
- Now run the following to count all Frostwalker I books:
- Result: Test failed. There are no Frostwalker I books.