In the release version of the game the item "Black Mushroom" is a component in creating the item "Staff of Herding." It is still an Easter Egg referencing Diablo 1 though, but also helps in crafting the item.
The full list of items needed to create the "Staff of Herding" are: Black Mushroom, Wirt's Belt, Liquid Rainbow, Gibbering Gemstone, Leoric's Shinbone, the crafting plans, and a 50,000 gold crafting fee. Additionally, the item can be "leveled up" per difficulty by finding the plans for that difficulty, using a created staff from the weaker difficulty, and paying the crafting cost to create the "leveled up" version of the staff.
List of requirements for the staff and its upgrades:
Staff of Herding: Requires Black Mushroom, Wirt’s Bell, Liquid
Rainbow, Gibbering Gemstone, Leoric’s Shinbone, and a 50,000 gold
crafting cost
Nightmarish Staff of Herding: Requires the Staff of Herding and a
200,000 gold crafting cost
Hellish Staff of Herding: Requires the Nightmarish Staff of Herding
and a 500,000 gold crafting cost
Infernal Staff of Herding: Requires the Hellish Staff of Herding and a
1,000,000 gold crafting cost
The total cost to craft the final version, the Infernal Staff of
Herding, is 1,750,000 gold.
It has also been confirmed that the staff is used to get to the secret level.
Farming bosses is pretty useless in D3 - you only get guaranteed rare loot the first time you kill a boss.
Besides that, I think none of the bosses is actually hard if you know the mechanics.
Best Answer
It's an easter egg, referencing the TV series Lost. The page numbers 4, 8, 15, 16, 23, and 42 are also part of the Lost reference. Basically, you can throw it away if you want to, or keep it in your private collection.