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.