I think the problem with leaving most of the Shar information to in-game books and such is that it's pretty likely that the player just...won't retain the information even if they did read it. There's a bunch of books around the game, all with all kinds of info, most of it unrelated to the main story. Why should the players do more than think "oh, this story is neat" and just file it away in the back of their minds never to be touched again? I'm all for expecting more of players, but for new players, context needs to be given. Regardless of Larian's intepretation of Shar, they should give us a concrete sense of who Shar is in the setting so that we can properly roleplay our characters. It's as simple as this; we should know what our characters know. If our character is expected to know who Shar is, and the dialogue options for the reveal imply that they do know, then we should know as players. ESPECIALLY because this is tied to the story of a major character. Larian is still telling a story, and if they want this reveal to land, then they should make sure we have the information needed to give this reveal an emotional context. This is a story, you do actually have to give players clear information sometimes so they know what's important and what to care about.
Here's the problem, your first sentence, and then your last sentence here. They do provide cursory information. The problem is that the player can, and in this case likely did, fail to make the connection between "evil Goddess that wipes her followers minds" and "that can be problematic". How deep do they have to dive into any specific lore topic to make it clear? Retention of information, and actually using what is provided, aren't the developers mess up. If nothing was provided, sure, but some information is provided. If someone is "speed running" it, and misses out on key things, that's on them. Note here that I'm not talking about an actual "world's fastest clear" here, but as with my example from swtor above, where players skipped the cutscene that explains what's happened on the planet, and didn't understand why the planet was essentially dead. So for this, things like "I don't have the time, or desire, to read the books provided in game, if I wanted to read, I'd hit Google", or similar. I'm left with the impression that we're going to need little signs at the "entrance" to various areas, explaining everything that the area is about, and some players would be complaining about all the exposition.
Again, at some point, yes, a player is going to have to do some footwork. Whether that's searching for lore books in game, or outside of the game on their own. This may come as a shock to some of us here, but that's why Wikis for these kinds of games exist. When I started in the TES series, with Oblivion, I had no idea about the lore. After over 1000 hours in Skyrim, and something close to that in ESO, I still don't. But I learned enough from the books that I've read in Oblivion, Skyrim and ESO that I have a handle on what's going on, and if anything comes up that I just don't understand, there are resources where I can look it up. Ironically, those resources are based on what other players found, in game(s) that I either didn't play, or that I missed in one of the games I did play. They dug a lot deeper into the provided lore than I did. The problem with doing something like this here is that there is 50 years of lore behind this game. If this is confusing a new player to the franchise, not just Baldur's Gate, but DnD in general, they're in for a really bumpy ride.
I'm someone who generally likes to read lore books in rpgs, I don't mind doing reading in RPGs at all, but this is a major part of the story we're talking about. This is different from skipping a cutscene. A cutscene specifically is meant to convey information that's probably going to be relevant to the plot. What other way do devs have to reliably convey information? But I as someone who LOVES crpgs, have been trained to think that ingame books aren't going to include things that will be necessary to appreciating major plot points. You're right that at some point the player has to take responsibiilty, but where do you think that point should be? I think that conveying information that's important to allow a major character twist to be impactful through a medium that in general conveys only extra information that doesn't impact the body of the story otherwise isn't where that line should be drawn. To give another example, it would be like in Mass Effect 2, when you find out that Mordin was part of the team that created the Genophage, but until that point the only way to know anything about the genophage would be if you'd read letters and popups around the game. Not only might you miss them, but just reading that information won't get across the emotional and narrative weight you should be feeling.
