Normally I'd say this is supremely irritating, but then I remember university days, and searching through books for someone else's notes to short-cut research. I didn't always work, but it had its moments.

I have occasionally checked out books from the library which had other people's writing and notes they've written, and I've often wondered whether the previous borrowers of said books should have known better, knowing that the books are not their property.

As for scribbling, well, we adults should be a bit beyond that. And those who have kids (of "scribbling" age) in the house should be responsible enough to watch their kids more closely.

I may be wrong, but I don't think a mature person would do something like this.  A library is the wrong place for an individual that would do something like this.

Although when I was in school I did occassionally find some of the scribblings helpful.

You shouldn't do this, although I HAVE seen some hysterical comments in the margins from time to time.

It's just about respect.

This would irritate the hell out of me, if I still relied on libraries for reading material. I hate it when people disrespect books, regardless of the content. The written word is a wonderful thing, and should be respected by all fortunate enough to be able to read it.