Marginal Note

I have discovered a truly marvelous demonstration of this trope that this margin is too narrow to contain...

Bull. This is a wiki. You have all the room you need.

Notes on a margin of the book, or close equivalent. Some teachers encourage doing this, most librarians discourage doing this. Of course, these can also be used in many ways as literary devices.


 * As the author's note, may be a part of Paratext.
 * In case of an In-Universe book, either may be used as context for another, or they may combine exposition and plot- or character- relevant texts in a more condensed form. Or to implement some forms of overlap between in- and out-of- Universe, such as Direct Line to the Author setup, if the author's book is supposedly annotated by the characters.

Contrast with Footnote Fever.

Paratext

 * Ghost in the Shell has some notes from Shirow.

Fan Works

 * In the short Harry Potter fic Surprise by Meteoricshipyards, Harry's great-granddaughter reads a document written by her great-grandmother which has handwritten comments by Harry in the margins. Counter to what might be expected, Harry's notes have little if any bearing on the story.

Literature

 * In Harry Potter and The Half-Blood Prince, Harry finding a potion handbook full of helpful annotations in the margins is an important plot point.
 * One sidebook in the Harry Potter universe, "Fantastic Beasts & Where to Find Them" has handwritten notes in the margins by Harry, Ron and occasionally Hermione, for extra comedy value.
 * The novelizations of Yes Minister reveal that Sir Humphrey is so accustomed to making marginal notes in his official documents that his personal journals are custom-made. They are tall and narrow, giving him nothing but margins to be written in.

Paratext

 * Stephen Colbert uses these.

Tabletop Games

 * The Dresden Files RPG rulebooks have notes by Harry, containing many Shout-Outs (up to and including references to "that other spellcasting Harry's book").
 * Some Warhammer 40,000 have such notes to spice up in-universe documents.
 * Only War has a tactical manual for simple Guardsmen, which is not all bad, but on the average, falls somewhat below half truth. It's supplemented by comments and more practical advice from its previous owner (presumably, deceased).
 * Shadowrun supplements (or parts thereof) are often presented as (mostly) In-Universe documents posted in electronic form on a runners' forum/bulletin board, with annotations and comments by the various users who've read them. These annotations can range from anecdotes disputing (or expanding on) the official description of various products, to Flame Wars between the users, to subtle running storylines.

Web Comics
"The boxed text contains the maxims themselves. The text below that is "schlolarly commentary" which, as suggested by the excerpt above the image, is going to be all over the map. The red-pen notes are from Karl Tagon, who acquired this book as an enlistee in 3044. His sergeant at the time told him he should use it as a journal of sorts, and so we'll get an unordered series of snapshots of his military career. Paging back and forth to put the notes in order will be part of the joy of having this in hard-copy. The blue-pen note above is from Alexia Murtaugh, to whom Karl loaned the book. (Well, "will-have loaned." That bit of story has yet to appear online.) While the book is in Murtaugh's possession it is going to get picked up and scribbled in by a few others, including Sergeant Schlock. We will leave room for you to write in it yourself, of course."
 * Schlock Mercenary has book The Seventy Maxims of Maximally Effective Mercenaries defictionalized as extra material. It was available with extra layers of annotations :

Real Life

 * The most famous marginal note is, of course, "…I have discovered a truly marvelous demonstration of this proposition that this margin is too narrow to contain".
 * "Everything was not like this" — written by ace A. I. Pokryshkin, 3× Hero of Soviet Union on the margin in his copy of "Soviet air forces in the Great Patriotic War 1941–1945" book. (per Timofeyev A.V., Pokryshkin, Chapter Ⅹ) Specifically, the part about fighting in April 1943 over Crimean station, where supposedly action had decisively offensive character, with large groups of fighters chasing off or blocking the enemy, well organized interaction between different air units, and so on. In that he was there, and it wasn't pretty at all, even a handful of aces thrown in could only help so much.