I'm afraid you can't protect an idea, because it's not possible to have copyright in an idea, only in the way you express it - the form, words, structure, and so on.
The best way, I would have thought, would be to write the book, developing it to its fullest potential, then try to find an agent, who would (you hope) find a publisher. Then at least
your expression of this idea would be safe, because you automatically have copyright in something as soon as you write it down.
The Society of Authors could advise you more, and they have very good booklets on such questions that you can buy:
http://www.societyofauthors.net/index.php4
Emma
<Added>Another option would be to self-publish it, but you'd need to be very confident that you don't need (or can find) all the input that a publisher has on editing, design, marketing, publicity, sales and distribution. That's a tough call, and the vast majority of self-published books reveal only too clearly (if you can track them down at all) just how hard it is to do all those jobs as a non-specialist.