This is the most comprehensive explanation ever attempted about the science of audiobooks vs. traditional books to this date. And even though this was driven by relentless curiosity, research, and passion for analysis, I might be wrong in some points and future evidence may prove that. This is a personal investigation to be taken by a grain of salt and not as the ultimate truth. The key concept presented, Learning‑to‑Exertion Ratio (LER), is a new metric to measure learning efficiency for the individual learner and is the main apportation of this guide. It captures the effectiveness and efficiency variables present in reading and listening to help learners find the best modality for them, whether that is books, audiobooks, or both, to maximize learning and minimize fatigue. This combination of characteristics helps to maximize the amount of knowledge you can gain, specially over the long term, for your personal goals, study, or curiosity.Creating this guide has been a personal passion for me that has helped me to understand this subject more deeply and to optimize my own learning for my knowledge journey and I hope it does the same for you because all of this was created for you, the smart guy who relentlessly wants to find complex answers to, seemingly, simple questions and don't stop until that hunger is satisfied.Note: The section about the mathematics of LER is completely optional and, if you like, you can skip it.