[2020] Today marks the 4th year that this perk was added and everyone STILL doesn't know what it does.

How very mysterious

I actually added another effect to it within the last 12 months.

I get that you're mad, but I feel I should address a few of these points:

• I put a way to read the bookcase in the Fight Cauldron into the game because a growing community of players were searching for it, and trying ever more elaborate methods to unlock it. There seemed to be a positive community building around that search. Rather than telling them to give up because what they wanted to find didn't exist, I wanted to give people what they were looking for, so there'd be a pay-off for their efforts.
• That meant writing and approving some special lore to discover (in my own time, because my working hours were dedicated to the next project I'd been assigned after The Brink of Extinction). This lore turned out to be, among other things, the first mention of a specific elder god other than Jas, and her name (Ful).
• This addition took a little approval from narrative peeps, testing and localisation time. Which meant that by the time I could get the pay-off live, the obvious solutions had long since been exhausted.
• Consequently the unlock method had to be very obscure; if the unlock condition became something that had already been attempted, failed and ruled out, that'd be disheartening. In fact, the solution I was originally going to ship was guessed in its entirety while the project was in QA, and I consequently had to scramble in some additional conditions (including the obsidian shards).
• Needing a specific quantity of obsidian shards equating to current runedate in your backpack is very obscure, I'll agree, although it is information available in-game (via the captain's log from ports).
• To help foster and encourage that hunting community, I regularly engaged with them on the forums via riddles and clues to help nudge them toward this improbable set of conditions.
• I'm sorry to hear that even now you feel frustrated about the experience; for what it's worth, I had the best of intentions.

Pretty much how it is now, excluding some additions and modifications (mainly the things that are a bit of a nuisance, like the gradually increasing shards, the TokKul-Zo needing to be uncharged, and I think also the Fight Cauldron coffer buff).

I'll ask the team and see if we can give some kind of extremely vague clue as to what is does

afraid not :(