Totally agreed. I've never understood why people are so opposed to marginal annotations - it's part of why the book format succeeded as a technology! (i.e you can't marginally annotate a clay tablet). Incidentally, I always loved how Tolkien's maps had these pointing hands to indicate significant bits, and only after reading a lot of old manuscripts and incunabula in grad school did I realize that he was copying the favorite annotation of medieval writers, the manicule: https://en.wikipedia.org/wiki/Manicule