Writing an ebook is the same as writing a hard cover. The only difference is the method of distribution. Determine what type of book, fiction, non-fiction, etc.., then come up an idea. It could be as simple as How to make a peanut butter and jelly sandwich to a romance turned sour. The only limit is your imagination. A good story has a good plot and believable characters. Make sure you research, especially if you are writing about places you have never been. Your readers will that the Eiffle Tower is located Champ de Mars and not on Mars street.