I have heard a lot about Price of the Phoenix (which I haven't read yet) but it seems like nobody ever talks about Triangle, by the same authors. It's a terrible book actually! But, as far as I've seen, it's the closer I've seen to an actual Spirk relationship in beta canon. One could easily argue that it *is* an actual Spirk relationship.
The gist of it is that both Kirk and Spock fall in love with a woman named Sola, and she falls in love with them as well (hence the title of the book. Most specifically, IMO, it refers to the bonds between the 3 of them). Instead of the usual love triangle jealousy, they soon get to a point in which they consider the relationship to be the three of them, not just any pair. And there's a heavy implication that the relationship between any pair of them is of the same nature, including Kirk and Spock's. It reads way more as a closed, complete triangle than as a relationship between Sola and each of the two men.
These are just one of many examples of Kirk and Spock being linked (or bonded) in a way where they can perceive where the other is, or how he's feeling. In this case, Spock raises his shields so that Kirk can't feel what Spock feels. (For context, this is before Sola is introduced, and the book happens post-TMP)
Once Sola appears and it's clear it's a love triangle situation, they both (Kirk, in this specific example) try to get her to choose the other. Specifically, in this case, because Sola has managed to do to Spock what Jim has tried to do for years. (Also, fun fact, she just banged Spock in the previos scene. I don't think Kirk is referring to that, but rather to "opening up to him" in that way)
After some point, the tone of "no! choose the other one!" shifts into "it's the 3 of us". Here Spock is referring to all the bonds between the 3 of them, not just their bond with Sola.
When faced with having to choose, she knows that neither of them would forgive her for not saving the other one, and she concludes that Kirk and Spock are the other half of each other.
Up until here, the "all the bonds between them are equal in nature". This passage is the first to convince me that, actually, they were intended to be. The antagonist specifically talks about the love not between 2 of them, but between all three. Reminder that Sola's relationship to each of them is already explicitly romantic and sexual.
And, after this moment, the book keeps reiterating it:
(Example of this reiteration. Kirk confirms that it's the three of them as a single "unit". They talk about threes quite a bit, actually. It gets pretty boring lol)
Going back to Spirk, Kirk thinks that nobody as good as Sola wouldn't felt what she felt for Spock. And yeah, tbf Sola falls pretty cleanly in the Mary Sue category, but I feel like Kirk is comparable in terms of their "caliber". Also: he (Kirk) had wanted that Spock :)
This is not a title drop, but almost: here's the triangle between their bonds. Again, triangles have 3 sides, and no distinction is mentioned regarding Kirk and Spock's side.
Bonus: the words t'hy'la (nor pictured here) and premise both make an appearance :) and so do the silver birds from the unused drafts of the "The City on the Edge of Forever" script
So yeah. What do you guys think? I don't think it's a good book by any means but, if any canon Spirk exists, this has to be it. They are in a relationship (even if, technically, they don't define it, this is a relationship. Also this all happened in like idk 24 hours), and this relationship reads as a closed triangle way more than as Sola having two partners.
Oh and, as a final disclaimer, I read this book in Spanish. The translation was shit, sometimes skipping random paragraphs and such. These are just the passages that caught my attention in Spanish, so I went to my English copy to screenshot them. I'm slightly worried that having read a bad translation of each of them before reading the official one has skewed my perception of any of them ^^;
