Why is there no 6 in the book of forests?

The book of the forest has no 6, because you haven't unlocked it yet. In the game Genshin Impact, you can unlock the sixth chapter of the Book of Forests. The following is the unlocking method:

1. First, we open the map and send it to the transmission point at the upper left of Wu Yu jungle.

When you arrive, go straight east from the delivery place. When you reach the marked point, click "Dig".

3. After the final excavation is completed, Chapter 6 of the Book of Forests will be successfully unlocked.