My Merriam-Webster defines it as "1: marginal notes (as in a book) 2: nonessential items." To my delight, I ran across a scholarly study of marginalia, Used Books: Marking readers in renaissance England, William H. Sherman (U. of Penn Press, Dec., 2007). The catalog comments:
...indispensable, offering and demanding a complete revision of standard notions of reading in favor of a much more capacious concept of the 'use' of books before the modern era... (Stephen Orgel, Stanford University)This attitude views the notes, doodles and jokes written in books by past readers as a sort of archeological trove illustrating both the life of the book and the life of the readers. I had known that scholars in older times often wrote in the margins arguing with the author of the book. I don't think I had run across anybody so delighting in excavating these marks.
In a recent sale catalog, one bookseller apologized for the condition of sixteenth-century volume as 'rather soiled by use.' When the book was displayed the next year, the exhibition catalog described it as 'well and piously used [with] marginal notations in an Elizabethan hand [that] bring to life an early and earnest owner:' and the book's buyer, for his part, considered it to be 'enlivened by the marginal notes and comments.' For this collector, as for an increasing number of cultural historians and historians of the books, a marked-up copy was more interesting than one in pristine condition.
They illustrate the catalog with "manucules," the drawings of hands with a pointing finger that we have all seen used to draw attention to a point or place. They also have a rather forlorn and disrespected label from a library that originally stated, "Marking of books is Forbidden." It has been variously changed to "Making of books...," "Making funny jokes is forbidden," and "making funny jokes in books is not forbidden -- as is irony."
Poking around on the web, I find earlier books on marginalia, Marginalia: Readers writing in books, by H.J. Jackson; and Romantic Readers: The evidence of marginalia, by the same author. In my catalog, besides the first Jackson book, I find, Marginalia by H. P. Lovecraft, collected by August Derleth and Donald Wandrei, and several poetry books with that title. So, I guess people have been studying it right along for a while. I just loved the Penn catalog's presentation of the book.
I have often enjoyed graffiti that runs along, replying and building on earlier defacements. And I will confess to following those silly page directions one finds in K-12 math books for some reason (go to p. 11, then on page 11, another direction leading finally to some really profound thought like math is sooo boring). But I don't like to find marks in library books, or even used textbooks because they are usually so asinine. But the idea of excavating even asinine comments for meaning is fascinating. Wikipedia notes, in its article on marginalia that there is a specialized term for scholarly notes in the margins, "scholia." They note that marginal notes can add value to a book,if done by somebody either celebrity or scholarly, or detract from the value if done by somebody who fits in neither category. And they supply a link to the wonderful pages by the library at Cambridge University where they catalog and demonstrate various damage to books, titled "Marginalia and other crimes."