ID:1994722
 
I'm in the process of making an interactive story game (however game-play mechanics don't rely on it.)

I'd like to include a lot of backstory into the story, which will be optionally learned for the most part. How should I go about feeding the information in-game? Perhaps books? I really don't like the idea of the player having an insane amount of cut-scenes or learning ALL info from dialog with main NPCs.

It may be useful to know I plan on the game being single-player to allow massive modifications of the world without worrying about ruining other players' games.
One way to do is to have random NPCs you interact with say things that give a little background to the particular area. RPGs will do this a lot where you will, for example, walk into a barren, torn down area and when you speak to the NPCs, they'll say things like "This land hasn't been the same ever since the evil Lord Diablo ravaged our sacred city 100 years ago." Pokemon does this too. For example I think in the 2nd gen Pokemon games, when you talk to one NPC within the first route, he says something along the lines of "I'm waiting to catch a Pokemon that only appears during the night." This teaches the player "the Pokemon that you encounter vary depending on what time of day it is" without going through the trouble of making an entire cutscene around it like they usually do with teaching the player how to catch a Pokemon.

Another way to do it is to make things obvious. Dragon Quest 8 did this. So the first main antagonist Dhoulmagus had a habit of making a crap ton of thorns appear wherever he caused trouble, and you learn that in the opening cutscene. This allows the player to immediately associate thorns across buildings or the ground with Dhoulmagus's sorcery. So if you go into an area and see thorns everywhere, it conveys "Dhoulmagus is responsible for the destruction of this place" without ever having to see another cutscene or read another line of text.
Konlet wrote:
I really don't like the idea of the player having an insane amount of cut-scenes or learning ALL info from dialog with main NPCs.

And you shouldn't. Exposition should be fairly minimal in regards to what is forced on players. If they are interested in the world, story, and setting then let them go out and explore it themselves to discover more. In-game texts and NPCs are a fantastic way to do this, but keep in mind that not everything has to be spelled out directly even then. You can give strong hints to players, or let them draw conclusions from limited information.
I think having a character who learns about the world with the player would be nice. A lot of stories just assume the main character knows about X, Y and Z, dialogue going accordingly. I think it's much more interesting and easier to teach when the character knows only as much as the player themselves do. That way, the story and knowledge can be gained at a natural pace, without requiring exposition.
In one of my favorite RPGs, Interplay's Lord of the Rings, you could interact with NPCs by talking to them in a simple window. You could type things like names to get information (if they had any), and "News" would often give you useful info. Some of it was world-building gossip, and some was where they told you about something they heard or saw and where--leading you to a side quest.
Popisfizzy hits the nail on the head here, the amount you force onto players should be minimal.

A good story never makes you play a bad game.

A bad story can have a negative impact on a good game, unless the story is non-invasive or completely optional.
Also check out Lost Legends by Woo. It's a pity that game never got completed, because it had a great deal of potential. The NPC dialogue is really good, giving you a strong feel for the world.
If we're providing examples of games that fit this well, I'd suggest Morrowind. In addition to having one of the most unique settings of any fantasy game ever, that game is chock full of lore out the wazoo, but very little of it is forced upon the player. It—and The Elder Scrolls series in general—is also a good example of not giving a privileged point of view: every detail comes with its own bias, and no single fact as provided should be assumed as the "correct" interpretation. It makes for a very interesting approach, I feel.

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And for a non-videogame example, there's Cowboy Bebop and its approach to the history behind the setting. Almost nothing is laid out in full, and the viewer has to piece together the parts to really get an idea.