Book upgrades are the way to progress in the game. I tought that the visual effects on the page were clear enough to give a clue about its importance, and It really have a gameplay effect that makes the game progress faster. I will not throw a lot of info to the player about the calculations made under the hood, this kills some of the magic, and i will risk turning the game into a math class.
About slow progress, another person talked about this already, but as a game developer i need to find a middle ground between a game that ends in 5 minutes of gameplay and a game that everyone stop playing after 5 minutes because It is too slow. Thats very, very hard. And every person playing has a tolerance of grinding, so its like cooking or making a movie: impossible to make everyone happy