I'm not a ridiculously healthy person, but I (knock wood) tend to have sick days left over at the end of the year. (Sometimes other days too, but that's a different problem.) I had been thinking a time or twelve that you should be able to call in book. There was that one time I arrived at the metro stop with four pages left - arggh! But even sometimes, you may still have chapters left to go but you arrive, or reach the end of your break or lunch period right as the heroine gets kidnapped. (Yes, I use this example a lot. It happens a lot. In books, that is.) Or right as someone was finally about to confess something. Or just something that has occurred in the story that you know you are going to spend the next batch of hours thinking about getting back to it. Now sure, as with many things, sometimes the anticipation makes it better. You appreciate the story more for your time away from it. It's the carrot at the end of the stick. But sometimes, you know you will be useless until you can get back and find out what happens next.
So, I had been thinking that you should be able to call in book to work. But the challenge I realized was that in a given year, I read a lot of good books. And while, given the average reading statistics for the population it probably (sadly) wouldn't be a challenge for the workforce, it might impact my ability to remain a productive member of the workforce. And someone with money to buy all those books.
But the solution is sick days. (Although if my company wants to allot me a separate bank of book days, I am all for that.) I think, much like the mental health day - wherein I believe some companies (check your policy) allow for the fact that sick days are essentially unplanned absences that might occur for other reasons, including I-just-don't-feel-like-it-itis (nasty stuff) and that, within a pre-arranged set of parameters, it might be allowable for you to not work for reasons other than crippling illness. So, if we expanded this definition to include calling in book, we'd be fine.