The Van Rijn Notebook is a pivotal element in the plot of Girl Genius and contains (among other things) notes written by master-Spark Van Rijn himself regarding the construction of his Muses. In the course of the plot, it has additional pages added to it with content from authors other than Van Rijn.
Appearances in the Story
Agatha first encounters the notebook during her stay with Master Payne's Circus when Moxana. Moxana has in hopes that it can help Agatha repair Moxana and/or build a replacement for her lost sister Muse, Tinka. before being distracted by Da Boyz visiting her for the first time.
Agatha later says that she does finish reading the book during this time period, but it isn't seen or mentioned again in the comic until after she is kidnapped/detained in Sturmhalten, when either Moxana or Tinka gets it out of Agatha's wagon and (and themselves) to Tarvek, whom they now recognize as the Storm King; he says he will use it to repair all the Muses. This all happens before Agatha gets a Lu-copy implanted in her head and the resulting Battle of Sturmhalten. The book next turns up two and a half years later among the of the fugitive Lady Selnikov following her reportedly using the book as a guide to ransack through the Corbettite vaults, which led to her fatal encounter with The Beast. It is by one of her pursuers that she was being hunted because she stole a "book and key" from self-proclaimed Storm King Tweedle or at least his people, but it's made clear nothing we hear about how or why she has it can be trusted. (However, see below regarding the key.) Agatha that the book is thicker than before; content been added by unknown persons. She is then once again distracted before she can examine the book further.
Violetta lugs the tome around until Agatha finally in Paris, after hearing a from Drusus Beausoleil about a "lost notebook" being sought by members of Tweedle's family (largely because it may contain information on how to free Mechanicsburg from the Take-Five time-bubble.) She quickly discovers that the book's new content is a collection of works originally written down by a wide variety of individuals, including Tarsus Beetle, Tarvek, the ancient Greek Meton of Athens, and Agatha herself. One of the first pages she reads prominently includes a depiction of the Enigma, called here The Muse of Time and drawn from memory after the writer/artist's encounter with the entity; this particular writer is not identified, but the page contains references to both time and pie-eating, things previously associated with Agatha's ancestor Robur Heterodyne. Also depicted is a Samophlange, as the device appears in the video game Warcraft.
When Agatha offers to hand over the notebook for safekeeping to the Immortal Library of the Grand Architect under Paris, it is to contain secret notes only readable when viewed in a light of the same wavelength as the "King's Lightning." (Presumably a reference the Storm King). Agatha and the library staff then subject the tome to an extensive battery of tests, further codes within codes, hidden away by Van Rijn. Still, in Agatha's estimation at least, none of the new information is worth the uproar the book has provoked; there is certainly nothing that will help with Mechanicsburg. It then occurs to her to connect the book's metal-fringed cover to an electric current. This causes the pages to glow and split, revealing yet more information about the Muses, along with schematics for a very complicated lock mechanism. This leads to an examination of Van Rijn's epic mural/fesco The Triumph of the Storm King, which prominently includes paintings of the Valois army, a (slightly damaged) depiction of the Muses and what appears to be the Storm King himself.
The book also appears on card #1 in the card game Pairs which features all the Muses, with the mysteriously named La Clef du Roy (The Key of the King) revealed by Agatha to be the same key that Lady Selnikov stole along with the book. It turns out the key allows access to Van Rijn's personal hidden "Hermitorium" inside the Immortal Library.