An ash covered tome is an item purchased from Petrified Pete's Ore Shop by the entrance to the Volcanic Mine on Fossil Island for 40,000 points. Reading it is required to unlock an enhanced Fertile Soil spell, which gives the effect of ultracompost by casting the spell while having two volcanic ash in the inventory.
When reading the book, the chatbox will state The book contains the secrets to an upgraded fertile soil spell, only available to those with access to lunar spells. There will then be a confirmation message, and if the player selects Yes, the chatbox will state You read the book and obtain the power to use an upgraded fertile soil spell. You must carry at least 2 volcanic ash in order to empower it.