Leeroy and Joan? (3)

The Yellow Book.

Not to be confused with the 1890s publication of that name, this is a generic term for a species of magical grimoire / notebook kept by Yellow Sign adepts. Such works are individual, but typically contain information giving +2-20% / + 1d4 ranks King in Yellow Mythology and +1d6-3 percentiles Cthulhu Mythos and costing 1d8 SAN (this for the first time one is read: the benefit for reading later books is less, quite possibly zero, as they have much in common), 1d6-2 spells such as Contact King in Yellow, Song of the Hyades, Summon / Bind Byakhee, etc. Spell multiplier 1d4+1, representing how clearly or cryptically spells are described. All YBs contain the Yellow Sign, on the outside or inside of the front cover; not all have yellow binding.