The Mushroom Book is an Event Item in Riviera: The Promised Land. It is obtained from Claude, after returning from Lacrima Castle. It is used to identify mushrooms for the upcoming stage: Nelde Ruins.
Acquirement Method: Talk to Claude a few times, and he will give it to you.
Function: It is used to identify the different mushrooms in Nelde Ruins, altering their descriptions in the Event Item inventory and affecting what they can be traded for. Avoid the Mushroom Book if the Spiral Shell is desired.