The chapters of my story are prefaced by short excerpts from the journal of the future assassin of the benevolent king my main character was welcomed into the court of. Since they are not dated, would it be confusing to readers if they were placed in reverse chronological order?
I realize this may sound like a non-answer, but, it depends.
Let’s break into two separate pieces. The use of journal and other epistolary elements, and non-linear storytelling.
Epistolary novels are constructed out of in-universe documents. Most often you’re reading correspondence between characters, their journal entries, and other various relevant documents. To a certain extent the genre assumes that the entire work will be constructed from documents. Though, nothing stops you from borrowing the format when it’s useful to you.
One of the major strengths of epistolary elements is that you can introduce information that would otherwise be unavailable to the reader without resorting to an omniscient narrator. If you’re working in first or third person limited, these can be very useful for breaking that format’s limitations without actually breaking the format.
If the journal is providing the reader with vital information, then that makes it useful. If it’s being used to tease them, I’d probably recommend restraint. That said, a lot of deciding whether or not to use epistolary elements comes down to execution and exactly what you have in mind.
For example: If the journal is a confession of sorts, or an attempt at justification, which runs parallel to the main narrative, with the assassin working through his reasoning, and reflecting on his actions, there’s certainly potential.
The journal could also be used to create a sense of unease and dread, as it gradually transitions from something innocuous into a more menacing document, as your assassin collects information for their plan.
Epistolary documents can also allow you to including background exposition that simply wouldn’t fit in the main narrative, particularly if it’s something your characters wouldn’t think about or aren’t aware of.
Your assassin may spend time writing about the politics, or economic situation that drove them to act, while the main characters remain completely oblivious to the events taking place outside of court.
If you have a use for the entries, they have a place in your writing. If you don’t, you may want to reconsider using them.
Non-linear storytelling can work, but it’s much harder. Even with date stamps, it’s entirely possible your readers would miss that the journal entries were out of order. This doesn’t mean you can’t do it, just that it is much more difficult to juggle these, and it will probably lead to some confusion among readers. Notice all the conditionals there. I am not saying you cannot do it; just that this is more difficult.
Also, a warning: when telling stories out of chronological order, you run a real risk of not having an interesting story when the audience finally parses out the chronology.
Some time-shifting is, almost, inevitable. If you have multiple characters, in different places, at the same time, and you intend to follow both, then you can put them in whichever order, and see the results play out. That’s fine, and most of the time the audience can be cued in to what you’re doing fairly elegantly.
When you’re writing a story in reverse, things get much trickier. (To be clear, if you’re telling the story of the assassin in his journals, parallel to the main novel, you are telling a story in reverse.) So, for the moment, let’s ignore that these are part of a conjoined book, and just focus on that story.
If you are getting something valuable from disrupting the chronology, then it becomes a question of execution.
Breaking the chronology means you need to reevaluate pacing, and how information is distributed. You’re constantly in a position where your characters are responding to prior events that the audience isn’t aware of yet. Then they make their discoveries, events happen, and that information drops from the story, leaving them with mysteries that the audience already knows the answer to.
Now, having said that, this is something the epistolary format lets you cheat your way around. You can have a conventional, linear, structure, where your character is recounting events, out of order as they pertain to their current topic. They may start by explaining their reasons for acting, then discuss how they actually carried out the act, before moving on to the way they gained access, or the actual motivation behind their actions later. If the purpose is to hold back a revelation for why your assassin chose to act, that can be shifted to later in their journal. They know why, they just haven’t bothered to record the event.
Also, worth remembering, you’re not going to start your journal saying, “so I’m here to kill the king.” That makes it a very dangerous document for the (in setting) author. They may even hold off on mentioning their motive for acting until it’s almost too late. This could be foreshadowed. For example: If their loved one was killed because they were part of a cult, or a traitor, they may mention the loved one, even that they’re dead, but not why they were killed, until much later.
They may make mundane notes that appear to be benign, but actually serve an operational purpose. For example, talking about meeting a member of the palace guard could appear innocuous, but that could also function as reconnaissance data for where the guards are deployed, and potentially even weaknesses which would allow your assassin to neutralize them effectively, without immediately tipping their hand to what they’re doing.
In particular, the journal could be useful to demonstrate the character’s social engineering skills, without ever needing to step back and say, “but, they’re very good at manipulating people to get information.”
If you think the journal will be useful, then you should include it. You don’t need to include the entries out of order if you don’t want to. You can probably shuffle the chronology far more elegantly by allowing the assassin to record their thoughts somewhat out of order, and handle the transitions there, rather than actually breaking sequence. Just, remember, your assassin is a unique point of view character, like your protagonist, so ration their information accordingly.