First let's get the obvious out of the way: Gideon the Ninth and Harrow the Ninth are delightful books.
They both contain references to modern jokes and expressions, which I would ordinarily hate in speculative fiction. But somehow in this it works. I think it manages to work because the character of Harrow takes the role of the straight woman: she is the necromancer in a grim, death-fueled magical world where the god-emperor of mankind presides over a never ending war against horrors from space, and embraces the role entirely. It is because she is alternately shocked and exasperated when the cracks in this world appear that we are shocked and exasperated. She grounds the world.
Gideon and Harrow are mostly from the perspective of their titular characters, and I'll admit I enjoyed Harrow more because I'm far more personally sympathetic to her. But both are extremely enjoyable reads, which is remarkable in particular because of the extreme tonal shifts between the two books.
But, obvious out of the way, I think Harrow has the best example of what I'm realizing is one of my favorite tropes: a character relying on judgement of their past, hopefully more rational self, to make decisions.
It's a delicious trope. It's the "you have to just trust me" which I hate, somehow enjoyable because the person you have to trust is yourself. To take instances of it:
In Harrow the Ninth, after recovering from a delirium, the main character finds a long note from her past self explaining (in somewhat abusive language) a set of rules that she must always obey henceforth. The rules start relatively sensibly, and culminate in the absurd injunction to check whether a particular person's jaw has been replaced. All of them have an explanation, and discovering the reason behind each is part of what makes progress in the book satisfying.
In the Stormlight Archive, the king Taravangian's intelligence varies from day to day, according to a bell curve, due to a combined blessing and curse. But he had one, vastly improbable day of incredible intelligence some years ago, during which he wrote out The Diagram, a plan which he has ever since (uncomprehending, mostly) been following. The Diagram drives him to do some horrible things, but he trusts in his past self that it all will work out for the greater good.
The classical example of this, of course, is Memento, where due to anterograde amnesia, the character must obey notes written by his past self to himself.
A more fun case of numerous examples of this can be found in There Is No Antimemetics Division. Asynchronous is a named plot point; doing scientific research, picking up from notes you wrote to yourself, some amount of time ago. Really delightful.
It's found in TV Tropes as Notes to Self although, looking through their examples, I think I know what I like about the above. It is, as in most cases, basically implementation. Many of the notes to self given on the TVTropes page are just bad: too short, unclear, clearly absent serious thought into how to do this.
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