The first part of Book of Heroes is now out. It tells the lore of Jaina Proudmoore (the base Mage Hero). We’ll follow her story through 8 different missions, during which she will go stronger and players will learn more about her. Players will earn one Mage Pack for finishing it (Mage pack contains only Mage cards from the current Standard format – so Year of the Dragon & Phoenix).
Additionally, a Hearthside Chat video on Book of Heroes was released. You can watch it here:
The video says that new Book of Heroes “chapters” will be out roughly every month or so, and it hints that the next one might tell the story of Rexxar (Hunter Hero).