I once took a course in Indo-European linguistics. The example from (if I remember correctly) Old Irish was a gloss in a Biblical text. In the comment, the minister grumbled about how Jesus didn't have the difficulties the minister did because He didn't have to travel to remote congregations.

Often what we write in books says more about us than about the book itself--and that's an interesting contribution.

If I remember correctly (my favorite phrase as a senior) there's a passage in the Bible forbidding tearing out a page from the sacred text (I used to worry about this as a kid) but not an interdiction against writing on the page of the Bible.