Another aspect you have to consider is that you would need to protect your idea/material, if it's not copyrighted a company can essentially 'steal' your 'intellectual property' and there's nothing you can do about it. I'm not sure what Bethesda's take is on the distribution of litterature, but they're a gaming company, so I don't think they'd have much use for it themselves.
To pitch your idea in the most successful way you'd need to pitch to the right person and not an assistant/middle-man. It wont be easy to pull something like that off, it'll take some searching on your part, but that's business, it's also good for the executive as they don't get hounded by hopefuls from day to day (not saying you are, but look at it from their perspective). Honestly, if you're coming here for advice on how to pitch an idea to a company like Bethesda, well, if I knew I'd be doing it this second
You just need to remember that a company like Bethesda isn't looking for 'loose change', your pitch would need to be worth their time and interest, and you need to consider your idea carefully. Most game-based novels get by on free and public fan-fiction, you would have to bring something unique and special to the table in order for Bethesda to even consider giving you the rights to sell that content.
It sounds like a bust, but that's business. If you really think you have an idea that they'll be willing to buy/lease out to you, then put a presentation together, protect your property, and get some credentials. That's about all the help I can give you