Once you get used to it, there's no real readability differences between horizontal and vertical.
"Hand written note" is ambiguous between two meanings. I've never seen my students take their notes vertically if that's what you mean. Most of the post cards we get from Japanese friends are vertically-written. Hand-written notes on post-it notes are almost always horizontal in my experience. Letters are commonly vertical.
"College essays" written by students are almost always horizontal. "College textbook" is a little harder since 教科書 is used in my experience both for what we would call a "textbook" and what we might call a primary source. Older primary source materials are almost always vertical (often published by 岩波). "Essays" like in a literature journal are generally vertical.
Business documents are almost exclusively horizontal. Or at least they are in my spheres (university, realtor).
Government treaties appear to be vertical. At least that was my experience having to correlate the US-Japan tax treaty between the English and Japanese. I was slightly surprised that a contemporary treaty is written that way. But I think that goes to show the signaling that something is formal that vertical writing invokes.