Right now there is no way to create a subfile for an ideabook, but you can easily create as many other ideabooks as you like to organize your Houzz photos. Here's how to create a new ideabook:

When you find a photo you'd like to add to a new ideabook, click the "add to ideabook" button below it. Then, choose "or new ideabook" and type in the name of your new ideabook in the blank space to the right. click on the "add to ideabook"button at the bottom and the photo will now be in your newly created ideabook. Hope that helps!