I think the level of detail depends on the age level/subject/activity for the day. I can see how in elementary school, subs need to know about Johnny's special snack time, kids pulled for OT, or how Sally likes to hide under the desk. Secondary is a bit different. Most of my IEP information is stuff like, "Abby needs to have all assessments read aloud," which is handled by the sped department and/or my co-teacher.
I admit I'm one of those teachers that will only take a day if I cannot physically get out of bed (happened last year with food poisoning) or if I have arranged for a sub I know and like far in advance. I have 2 subs I really, really like and trust on my list and they know how things work in my room by now.
Do subs want to read a novel? Probably not. When I subbed, I was happiest when I had a clearly defined plan to follow for each class period. But the health alerts absolutely need to be available. I have a kid this year with a super severe peanut allergy, to the point where I won't even eat peanut butter sandwiches in my room for fear of triggering an episode. He carries an epi pen. It's VITAL that any sub in my room knows this, and this is not considered confidential as far as subs are concerned (at least by my district, ymmv). It is of course possible that the teacher you are subbing for has no health alerts/IEPs, but I would be shocked if that were the case.