Libraries
Marginalia and the Second-Hand Textbook
The notes in a used book are a record of how it was actually taught.
A textbook that has passed through several owners carries a second text written in pencil, and that second text is frequently more informative about the teaching than the printed one. It records which passages were dwelt on, which were skipped, what the examiner was thought to want, and where the printed explanation failed badly enough that someone had to write a better one in the margin.
Kinds of annotation
The marks fall into recognisable families. There is navigation: brackets, arrows, page references and the underlining of definitions, all of which exist to make the book faster to re-enter. There is compression: numbered lists written beside prose, and diagrams redrawn smaller. There is dispute: a crossed-out sentence with a correction, or a question mark that has clearly been left as a question. And there is dictation, where a whole margin carries a sentence spoken by a teacher and written down verbatim, usually because it was framed as the thing to remember.
The dictated margin is the closest thing surviving textbooks have to a recording of a lesson. It shows the gap between the book's account and the classroom's account of the same material, and that gap is where the actual teaching lived.
Why the used book was normal
Second-hand textbooks were not a fringe economy. Where books were issued rather than bought, a copy was expected to serve several cohorts, and the inside cover often carried a printed table for successive names and conditions. Where books were bought, the resale market was an ordinary part of a family's arithmetic, and the annual edition change that voided that market was noticed and resented long before anyone described it as a business strategy.
This has a design consequence that is easy to overlook. A book expected to be inherited is built and laid out differently: margins wide enough to write in, paper opaque enough to take pencil on both sides, a binding that survives repeated flat opening, and a front matter page designed to be written on. A book expected to be consumed by one owner in one year need not do any of this, and mostly stopped doing it.
The annotation problem in digital reading
Digital text made annotation trivially easy and inheritance almost impossible. Marks are attached to an account rather than to a copy, and they do not pass to the next reader. The second-hand textbook's accumulated commentary, built by strangers over years, has no straightforward digital equivalent, and the attempts to build one have generally foundered on the fact that most annotation is private, partial and only meaningful to the person who wrote it.
What does transfer is the deliberate annotation: a shared set of notes prepared for others. That is a different genre with a different quality, closer to a study guide than to marginalia. It is more accurate and much less revealing, because it records what someone thought should be noticed rather than what someone actually noticed while struggling.
Reading the margins
For anyone interested in how a subject was taught, a shelf of used textbooks is a better source than a shelf of new ones. The printed page tells you what a publisher and a committee agreed to state. The margin tells you what a room full of people did with it, including the parts they did not believe.