Far too many aspiring writers keep their manuscripts and stories totally under wraps, afraid that someone will take their idea and sell it themselves. There are four simple, excellent reasons why you should tell people about your book, however, and they outweigh any risk of IP theft.
Barring a few obvious examples, writing is a solitary enterprise. Even writers who live with other writers work out ways to keep themselves isolated while they are working, and the duos who co-author books don't usually do so from the same room. That's more of a practical arrangement than anything, though, as anyone who has ever tried to write while having a conversation can tell you.
In On Writing, Stephen King offers this great little nugget of word-sharing wisdom: "Write with the door closed, rewrite with the door open." When you're working on your first draft, it's best not to tell anyone anything about what you're doing, because they might point out that you have a lot of big, structural problems in your work, and that might tempt you to throw the whole thing out and start over. Once you've got 50,000 or 100,000 words under your belt, you'll be more inclined to fix what's wrong, rather than trashing months of hard work.
So you should tell people about your book. Here's why.
1Your Idea Isn't As Original As You Think
I hate to burst your bubble, but your idea isn't the groundbreaking, Great American Novel-worthy concept you think it is. I'm sure it's good, great even, but you have to face the fact that there are only so many possible story arcs in fiction. If your book becomes a smash hit, there will probably be more than one article written on how your story revives some long-lost element of another writer's work. That's just the way it goes.
Because of this, it's entirely possible that someone else is already working on a 30th century, interspecies zombie romance set in the Andromeda Galaxy. And even if what you share inspires someone else to write something remarkably similar, that's OK, because...
2Writing A Book Isn't Easy
Writing and publishing are hard work. It takes weeks, at best, to write a draft, months to revise it and find an agent, and potentially years to sell it and put it in stores. That's a really long process for you, as the writer, and you probably have some small inkling as to where your book is headed, unlike an idea thief.
Someone who steals your work is really in for the long con. The idea that they might try to recreate your story, based on a 15-words-or-less summary, is pretty laughable. Even if they do steal your summarized concept, you shouldn't worry, because...
3No One Can Write Your Book Like You Can...
You don't have to worry about someone stealing your book idea, because no one can write your story the way you will write it. Your book is so much more than the words you put on the paper, as I'm sure you know. It's all the scenes and details in your head that will never make it into the official story, but that are essential to your ~vision~.
Look at it this way. If I tell you that I'm writing a book about four sisters raised in a radical social experiment — which happens to be the story I'm working on right now — you could take those few words and turn them into quite literally anything you want. Even if I tell you that my story involves a Millennial lifestyle trend and a beloved children's book, you still have very little chance of coming up with my idea, and you certainly couldn't divine enough details to steal it.
4...But You Need Help Writing It
Please don't be this guy.
You cannot properly edit your novel without at least one outside reader to look at it and tell you what they think. As I've said before, if you are the only one looking at your words, you've put yourself in a vacuum, where you will not grow as a writer.
So share your ideas. Let people you know and trust read your work, and listen to their thoughts and opinions. Talk about your book as much and as often as you like, because no one is going to steal your idea. If anything, they might just make it better.