One of the things that immediately jumps out at me is the opening. Sure, it gets right to the point, but it's not really that much of an exciting hook. Think of the opening to your letter as being like the prologue to a book: if you don't grab your audience's attention from the get-go, you'll find it'll be harder to grab it later on. There's a variety of different ways to do that, but one of the best ways that I've seen is to open with a sentence or two about the work and
then explain what you're enclosing. For example, here's a paraphrased version of the opening paragraph to a pitch letter (same concept as what you're doing, just for magazines) I once wrote.
(And to answer anyone's question, yes, it actually got published. It's... probably better not to ask about what kind of magazine would accept this idea. The answer is, "It was really weird.")
Martha Stewart has made her name as a domestic goddess, so it's easy to trust any cookware with her name on it to be nothing but the best. However, when the power is cut and resources are dwindling thanks to post-meteoric geological disasters and climate changes, how useful can her pans be? In a 500-word article for your back-of-book review department, I propose analyzing the Martha Stewart Collection stainless steel frying pan for both how well it cooks and how well it can be used to defend your food caches from scavengers.
(Note: The actual thing was a bit clearer and went on for a bit longer, but you get the idea.)
In any case, there's a lot of things to note here. The first sentence is designed to grab a reader's attention by pushing them directly into the concept of the work. Just enough is said to introduce the reader without giving too much away. (The paragraphs following this excerpt would go into more detail about the work's premise and how it fits the other material put out by the publisher -- or in this case, the magazine.) There's also a transition (albeit it could be smoother) between the hook and the actual proposal, and really, there's various ways you could pull that part off. You could try a question, or you could simply say something along the lines of, "Deep within a world strongly resembling feudal Europe, the characters face a journey full of danger and intrigue. Sin, a [insert number here]-word/page fantasy novel, is a complete work that promises plenty of action and magic for (insert intended audience here, such as 'the young adult age group'). I have enclosed the first three chapters for your review." Only, you know, better worded because you know the novel better than I do. Not to mention I'm running on three or four hours of sleep and can't come up with pitch letter material worth crap at the moment.
Of course, other authors might tell you that it's okay to start off with just something along the lines of, "I have enclosed these chapters for you to review," and while that's kosher too, I've always been uncomfortable with how demanding it sounds. Imagine if some stranger walked up to you on the street and said, "Here's a baby. You take care of it now." So it's best to figure out a way to word things politely, rather than simply saying, "I've enclosed this thing for you." One of my favorite ways (that I've seen other people use, anyway) simply said, "I would like to submit my manuscript, [insert title here], to you." It still requests the recipient to look over your work, but it sounds like it asks for permission to have it be viewed, rather than sounding as if you're dumping your work off on them.
Another thing I noticed is that you've
started off by mentioning that Sin resembles a generic high fantasy. Here's a tip. Literary agents (and by extension acquisitions editors) will tell you one important thing about belittling your work: "Okay, so why should we look at it?" Remember, editors, assistants, and agents have a lot of manuscripts to go through on a daily basis. Not only do you need to hook them, but you also have to convince them to give your work the time of day. You're not going to do that by saying your work is generic. They'll simply respond with, "Okay, your work is generic." Then, they'll set your packet aside and go to the next author in line.
While this isn't license to be egotistical, you'll also want to avoid selling yourself short. It would be a good idea to dodge the entire idea of comparing yourself to other people anyway, given that (as I've said earlier) this can be a dangerous thing because of how quickly literary trends shift. If you acknowledge that your story sounds generic or if you attempt to say it's better than a similar story, then that probably won't mean much to an acquisitions editor because they're always on the lookout for the
next trend -- as in, something different but interesting. Highlight the good points of your work without leaning on comparisons to others (until you get to the point about why it would fit in with the publisher or agency, anyway). Don't say it's generic; say it's a new take on a fairy tale style of writing (or however you would define what you're doing). Be confident in the creativity of your writing, and make sure you go out of your way to sell yourself to a publisher by making it clear it's fresh and creative. It would also help to go into more detail about the plot in order to make the synopsis itself sound as creative as you set out to make your story sound; the details we get as it is are vague at best and actually advertise your story as being pretty generic. (Sorry to put it that bluntly, but there's a lot of stories about two kids going on a journey only to end up fighting against a tyrant. Heck, half the fics in this fandom are basically about that. What makes your story different?) However, note that some publishers or agents will ask for details in a separate letter (the query letter -- what Saber was talking about), but it would still be a good idea to reword your synopsis so that the premise still sounds new and interesting without losing its briefness, if that makes sense.
Lastly, one of the biggest pieces of advice is to link your novel to them. Remember how I said it's a bad idea to compare yourself to other people? Well, here's where you can bend that rule just a bit: you'll also want to gain the editor or agent's attention by convincing them that your work fits the kind of stuff they publish. Do your research and find similarities between books already on the list. Is it a high fantasy like a novel they've printed just this year? Does it contain a coming-of-age story like another book? Don't make it seem like you're simply rewriting those books, but
do make sure to clarify that your novel is a good fit with the house or agency. That not only helps give them an idea of what your book is about, but it also tells them that it might be easier to sell your novel than others because it's right up their alley.
Hope that's not too confusing, and hope it helps a bit. It would be interesting to see where you take this from here.