Here goes all the rush you had over the current book, because the new book suddenly looks like a much better project to work on.
I have this issue with my role-playing campaigns. I'm starting to write this new, cool, unique campaign, spending a month or so thinking about it, spending another month writing it, and then BOOM! A new idea takes over and the old campaign looks boring and trivial.
When playing D&D (or any other role-playing game for that matter), you can solve it by weaving that new idea into the current campaign. Most plots can be woven in, given some thought and consideration. Zombies have nothing to do in a political campaign taking place in the Elven Court, but if you really want Zombies dragging their feet all over the place, there are plenty of ways to do it...
But in a novel, it can be more difficult. Unless your novel concept allows such dynamic transitions, its going to be difficult to incorporate such changes in a way that will seem natural. A novel dealing with an aging police officer trying to cope with the death of his son will probably suffer from a change of direction that includes Aliens from Mars, especially if there was no indication that the book is going in that direction at the first place.
So what do you do?
Unless you want to write two novels at the same time, you need to do something to avoid your idea getting lost.
I would try the following:
- Think about how to include that idea in your current book. Just think, don't write anything into your book, just jot notes and ways to include it.
- If #1 is not possible, set aside your first novel for a day or two, and write down anything you can regarding your new idea in an "Idea Notebook".
- Go back to your first novel!
As you are writing your original story , you can go back to your "Idea Notebook" and skim it for inspiration. Maybe you'll find a way to incorporate that great idea of yours into your current story, maybe you won't, but your ideas aren't going down the drain.
But the easiest way to solve it is to write a book in which such Reality Shifts are natural. Exactly like what I'm doing in my Beast of Burden novel :-)