It's all well-explained at the PTO (Patent & Trademark Office) website.
You can trademark a name, phrase, title, image, etc. You can patent a product -- the schematic and design of a product.
I am not sure if you are identifying a "title" as an "idea" in your inquiry, BER! ? That's how it strikes me here -- that "_________ for Dummies" is what you are calling an
idea, but the PTO would call it a
title, and hence is registerable. The idea/concept/schematic behind the book's content is cannot be protected though, as I understand it, because it's a creative work. (This should be verified directly with the proper offices. I am not an expert on this.)
HOWEVER -- you
really need to do a search @ the PTO office and make sure that "__________ for Dummies" isn't already tradmarked. It might be the format of "whatever For Dummies" has been registered and hence you won't be able to use the
For Dummies moniker.
In that case, you'll need to contact the trademark holder (guessing it's a publisher) to ask them how you go about pitching your idea for the right to be included in their series.
As mentioned above -- I'm not an expert. If I have made a mistake,
please correct me.

Bailey