The Floating Spellbook loot card is redeemable for a Gusting Grimoire loot item, which teaches the use of the Gusting Grimoire pet.
The Gusting Grimoire will likely be very similar to the Lofty Libram, in both appearance and behavior.
Floating Spellbook is a super common loot card - found in approximately 1 in 11 packs of Betrayal of the Guardian.
Betrayal of the Guardian booster boxes are selling for around $100 each, which would put the average price of this card at around $40, were you to try and pull it from a pack.
On ebay right now you can buy one for under $10!