There's an interesting piece in the New York Times that I encourage you to read, about marginalia and its increasing value in the book trade. Three major universities have received significant funding to study marginalia created by certain people, the magician John Dee being one.Until recently marginalia was looked on as a sin in the book world, but these days it can be worth big money.
If you've ever owned a second hand textbook, then you already know what marginalia is. Whether or not the scribbling and doodles interspersed with the text is worth anything will depend on the identity of the scribbler. The Times quotes a Christies auction in which a chemistry text annotated by Michael Faraday sold for $38,000 when the high estimate was $25,000, and points out that an ordinary, pristine copy of the same text would sell for only a fraction of that amount.
Before now, it was common practice to destroy marginalia when it was found. Booksellers thought it made the text less valuable, calling marked books 'dirty,' and libraries routinely destroy this ephemera when it is discovered. There's an organization, Book Traces, devoted to tracking down and preserving these items and notes, wherever possible.
From a Bookhounds point of view, removing marginalia - if the characters want to do that - uses the Forgery ability. It's not just about erasing text; it's about making it seem as though the text was never there, a tricky thing to pull off. The Keeper might call for something like this if the Hounds pick up a text that needs tipping in to make it complete, and increase its value.
On the other hand, there are plenty of times when marginalia might add value, or become significant to the plot. Poems annotated by Randolph Carter, or an anatomy text with Herbert West's scribbles, are only ever going to interest Mythos scholars, but those can be the most determined collectors. Don't forget that many of these characters traveled extensively in their lifetime, which means that they could have left marginalia in all kinds of interesting places. Imagine, say, an official history of the Regiment with odd photographs, scraps of paper drawings and maybe a medal or two attached, or used as bookmarks. Say it was West's old unit in Flanders, and that West was one of those who contributed a photo or two, perhaps one of those drawings. Now imagine it being further annotated by West's old C.O. Major Sir Eric Moreland Clapham-Lee, the headless medico.
That kind of thing is bound to be plot significant rather than valuable in its own right. Though someone with Mythos knowledge might be interested in the contents, to anyone else it's just a battered old book. But there are plenty of characters in Lovecraft's fiction whose marginalia might be valuable in its own right. Occultists and scholars like Von Juntz, Ludwig Prinn, the Comte d'Erlette, and so on, are all going to be of interest to people who have little or no understanding of the Mythos, and those scholars will have left behind letters, notes, drawings, and other items, stuffed in the leaves of the books in their libraries. Picture a medieval Arabic text on astronomy, annotated by Von Juntz, or a love letter by the Comte d'Erlette used as a bookmark in a history of Paris. Those things may not have any plot significance, but there are going to be people out there who will go nuts over them.
Finally there is the double whammy, a Mythos text with marginalia. Something like the King in Yellow with annotations by Hildred Castaigne, or Leggett's 1821 Mysteries of the Worm with notes by Halpin Chalmers. The books themselves already add Mythos, perhaps potential Magic, even granting dedicated pool points. The King in Yellow, for example, adds 2 to Cthulhu Mythos and 1 to Art, permanently. One annotated by Castaigne might do all that, and also provide 1 potential Magic point, or, in a Dreamhounds game, refresh Instability.
That's it for now! This may be the last entry before the New Year, so let me take this opportunity to wish you every good thing in 2015!