From everything I've been scooping up about this, seems like its close to a multiplayer of Joe Dever's Lone Wolf gamebook series, which got many of my hours in the early 90s as a kid without a DM for miles.
Having played A LOT of D2e:Road to Legend, I'm inclined to think that they are vastly expanding the RPG elements that happen in the app. Which follow the model of:
Issue // 3-4 options with some disabled based on party comp // resolution of issue // store reference of resolution // invoke resolution reference later.
As an example:
My party ran into an old lady on our way to our next destination. Presented with talking to her but since we had a conjurer with us we were able to get a Mana Weave rune from her, player used mana weave and began to rely on it (cuz he sucks at rolling ), a couple of quests later the mana weave blows up after a surprise event between rounds and the witch is heard cackling.
The product shot showing multiple books makes me inclined to believe that we'll see multiple interweaving decision trees that reference player materials independently from each other. Its a design decision I would make to cut down on the what I used to do in the Lone Wolf books, find a really good optimal result and work my way backward!