First you click on "Ideabooks" at the top of your screen. Then you go to "Your Ideabooks", it should be under "More Ideabooks" on the left hand side. Next you select the one you want to delete and give it a click. Lastly, you click "Edit" and then click "Delete this Ideabook", this should be at the top of the screen. You can't delete multiple Ideabooks at one time so if you have more than one you have to go to each one and delete it separately.
Hope I helped!