First, ask yourself, do you just want some brain exercise or you want to open your eyes to know that there is nothing that you do not know. If your goal is the former, grab any book in the subject of your liking, definitely go for the popular ones, it will ensure the probability of your liking and your desire for making your brain tool more sharper and capable. If your goal is the later, then there is no book for you to read.
Choosing the later goal means coming to a realization that there is nothing that anyone needs to know and that the knowing and the know-er are the same. If the goal is such, then you must stop reading books, listening to ideas, and learning tricks. Instead, you must start unlearning the brain exercises that books, any thinking, imagination, or mental noises create. You don't require any book, knowledge, mental thinking, and tricks to come to a realization that you are the know-er and the knowing. You always Knew, no book will impart this knowledge. You must stop seeking for brain teasers and feel the all knowing know-er that you are, that is omnipresent, omnipotent, and omniscient.
Choosing the later goal means coming to a realization that there is nothing that anyone needs to know and that the knowing and the know-er are the same. If the goal is such, then you must stop reading books, listening to ideas, and learning tricks. Instead, you must start unlearning the brain exercises that books, any thinking, imagination, or mental noises create. You don't require any book, knowledge, mental thinking, and tricks to come to a realization that you are the know-er and the knowing. You always Knew, no book will impart this knowledge. You must stop seeking for brain teasers and feel the all knowing know-er that you are, that is omnipresent, omnipotent, and omniscient.