(via the svtfoe wiki)
I very recently started watching Star vs The Forces of Evil (no spoilers, please!) and was amused by an episode where Star needs to undo a spell she’s cast on Marco. She pulls out the wand’s manual, an ancient, crumbling tome filled with the wisdom of ages of wand users to consult, only to realize that all of their notes are so cryptic and poorly organized that it will take her ages to make any sense of them. This got me thinking about magical journals in general. A common staple of fantasy fiction is a magical guide to the world in question, typically in the form of some kind of handwritten diary or log. Sometimes a book is just a book; I can’t imagine, for example, that Newt’s finished version of Fantastic Beasts and Where to Find Them will be anything but a basic bestiary. That’s not necessarily a bad thing. However, these books are often most compelling when they serve a greater purpose than simply as a how-to or a reference of some kind. By including these books in a layered way, we can add additional complexity to the stories we tell.
John Winchester’s journal in Supernatural mostly exists as a plot convenience and a way to build Dean’s character rather than a guide in and of itself. While it comes in handy as a guide, we also see Sam and Dean gather plenty of useful information without it from sources like Bobby, libraries, and even the internet. Therefore, it’s not necessarily important because of its guide-ness, but rather, because of what it means to the characters. Dean receiving his dad’s journal at the beginning of the show is strange enough to him that it leaves him worried that something terrible must have happened to John—why else would he part with his most prized possession? Dean’s loyalty to his father is all tied up in the Winchesters’ use of the journal, and puts so much faith in his father’s creation that when he is faced things the journal says don’t exist, such as angels, he’d prefer to believe the journal rather than the divine evidence in front of his face.
(via the gravityfalls wiki)
The three journals in Gravity Falls also serve a much larger purpose than simply offering a guide to the creatures who populate that mysterious corner of the Pacific Northwest. Yes, Dipper’s book is certainly helpful at times in identifying and defeating various paranormal threats, but the real appeal of the journals is the larger part they play in the conspiracy of Gravity Falls. The secret behind who created them and for what purpose is a driving force of the series, and discovering a new piece of that puzzle is typically far more exciting than simply using the journals for their more straightforward purpose as bestiaries.
In Harry Potter and the Half-Blood Prince, the Prince’s potions textbook does ostensibly help Harry pass his potions classes, its base-level use. However, its more important role in the story is, again, in contributing to Harry’s character. We see him learn the dangers of putting faith in unknown sources when he uses the spells in the book without knowing their purpose.
(via hp wiki)
We see that he has once again underestimated the depth of Snape’s character by failing to put together a textbook annotated by a potions genius and his hateful professor. And we see him fall into the trap of hubristically believing he is hot shit because he’s the chosen one and also apparently really good at potions, even though he doesn’t deserve the praise he’s getting. (Harry Potter is actually full of these kinds of books, although these examples are less personal than a hand-annotated journal: The Monster Book of Monsters is a snarling, bitey testament to Hagrid’s overall inability to realize what’s dangerous and what’s appropriate for children, and the running joke that only Hermione has read Hogwarts, A History contributes to her characterization as the resident bookish smarty-pants.)
There are certainly other ways that journals and other similar books in fantasy can move beyond their literal bookishness, but these often turn out to affect the story in more dramatic ways. Tom Riddle’s diary, for example, becomes a character in and of itself—the main villain of Chamber of Secrets. The Death Note is much the same—without it, there would be no story, and Good Omens couldn’t happen without the characters figuring out the meanings of the prophecies in The Nice And Accurate Prophecies of Agnes Nutter, Witch. Interview With the Vampire reverse-metas itself, with the real published book by Anne Rice appearing in The Vampire Chronicles’s universe as a book by character Louis de Pointe du Lac. These are all fascinating uses of books in fiction, but they’re not particularly subtle—the story becomes about the book, rather than the book contributing to the story. By using these fantasy guidebooks in more subtle ways to affect character and plot, they pull double duty in by working as both a book and a story device, and can make both the worldbuilding and the characterization feel more organic and interesting.
It’s certainly always fine for a book to just be a book. All Moste Potente Potions ever does is show the trio how to brew Polyjuice Potion, and that works for the story. I’m also never going to complain about the plot of a story being entirely about a book; believe me, I’m a “the more the merrier” kind of girl when it comes to books. Both options have their place in storytelling. However, I find the more nuanced way of building character and story through journals and reference books particularly interesting. I love being able to draw inferences about characters from the way they interact with the things that are important to them, and these guides often tell us more about the person reading them and the person who wrote them than they do about the actual contents of the book. This is a really great and effective way to do characterization and worldbuilding, and I’m always excited to come across it in fiction.
Follow Lady Geek Girl and Friends on Twitter, Tumblr, and Facebook!
Hear more from Lady Saika on Character Reveal, the podcast she cohosts with BrothaDom!
Lady Geek Girl and Friends 
Possibly the best–or at least the most fun, and most appealing to my academic brain–use of books within a book is in Jonathan Strange and Mr Norrell. Not only do the characters all covet and use books about magic and magical history, but the novel itself uses footnotes to cite the magical tomes mentioned in the text, and expand on stories and information you would find in those books. It was a fascinating and delightful way to expand the worldbuilding and flesh out lore that wouldn’t fit in the main narrative, and made the whole concept of scholarly magicians feel so much more real in that the author provided a whole library’s worth of actual fictional texts they could be studying.
The modern existence of talismanic publishers (e.g. Scarlet Imprint, Xoanon/Three Hands Press, Ouroboros, etc.) is attempting, in real life, to do some of what you’ve suggested here: namely, to create books that are as important to those who own and read them as their most prized possessions, and works of magic in and of themselves. If one creates such a book oneself, it also has that effect, potentially…it certainly has for me, but to elaborate more on this would get into various strange corridors of my own spiritual practices and such, which may not be of great interest to you, so I’ll leave off there for now, and I thank you for another intriguing discussion! 🙂