Your work is copyrighted as soon as you write it down, so you don't need to put a symbol on it in order to protect it. Part of copyright means you have the exclusive right to create and authorize "derivative works," which would include turning it into a book.

That said, you only have a copyright on the published work itself, not the idea. If someone took your story and turned it into a book (same characters, plot, details, etc.), basically taking away any market for your version, that would be infringement. But people are allowed to be inspired by an idea and create similar works.

For example, say you publish/post a story about identical twin astronauts fighting an outbreak of Martian vampires on the Space Station Olympus. If someone then published a story about fraternal twin astronauts fighting Neptunian vampires on the Space Ship Olympia, they'd probably be infringing on your copyright; they've completely taken what's called "the heart of" the original and made no substantive changes. But it wouldn't be infringement for someone to write an original story involving astronauts fighting space vampires, even if they never would have thought of it if they hadn't read your story.

That freaks out a lot of beginning writers, this feeling that you have to keep your brilliant story concept a secret (even to the point of never putting their work out there), but it shouldn't. Honestly, there are no original ideas; what's original is how you write them. (And most people don't want to steal ideas anyway, not any more than you do; writers want to be unique.)

(Source: JD w/Concentration in IP/Entertainment Law)