Acting on player feedback, Blizzard will be turning the Manuscript of Endless Possibility into an account-bound toy, though new purchases will be restricted until the update is deployed.
Currently, the manuscript adds a new spell to the character's spellbook, randomizing the appearance of dragonriding mounts each time they're summoned until the spell is deactivated. But at a cost of 40 Riders of Azeroth Badge per character, it can be quite costly across several different characters, which the upcoming change aims to address.