Can be achieved by placing different Book style furniture pieces, it doesn't matter what type (can be kitchen, chairs, art) as long as it has the Book indicator in the game.
More Book style furniture will unlock while researching different things at the decoration shrine and you will also be able to unlock some Book style Furniture via friendship levels.
The game will state if you already have 1 of an item and that the stats won't go up, i.e.; have 2 of the same Book style furniture of Book: 2 stat, is not 4 stat, it's 2 stat.