So paper is made from trees, and trees are mostly made from two things – cellulose and lignin. The stuff in this image is lignin. And because plants actually rule the world, lignin is everywhere. excluding fossil fuels, most organic carbon on Earth is lignin.
When lignin breaks down, that's where that delicious book aroma comes from!
When the lignin in paper breaks down, it releases a bunch of small molecules – and many of those molecules also give flavour to foods and spices.
Four which often give their aromas to books include vanillin (the main flavour compound of vanilla), benzaldehyde (part of the scent of almonds), anisole (a warm, spicy fragrance found in aniseed), and furfural (also an almond flavour compound).
@InvaderXan I don't think that's what they mean by digesting a good book. (It does sound delicious, though, joke aside.)