What's happening in my brain when when I read an entire page or so of a book, only to realize that I wasn't actually paying attention and have no idea what I just read?

"Mind-wandering", an extremely common phenomenon, but understudied subject in cognitive and educational psychology. MW can take the form of task-related (e.g., imagining scenarios utilizing current material) or task-unrelated (e.g., dwelling on relationship issues, dinner planning, etc.). MW can be mitigated by reaffirming your internal motivations (e.g., interest in the topic) or external motivations (e.g., upcoming exams, being paid to absorb material). However, MW has also been correlated to higher creativity measures, so it ain't all bad. Source: cognitive neuroscience researcher.