As others have said, inspiration is not what makes a writer. Sitting down and writing when you aren’t inspired is what makes a writer.
And be carfedul of “creativity” books. Most often, they’re just a way to let someone pretend to be creative, rather than letting them actually get the work done. Writing a letter to Elvis might be fun, if you want to be a writer who writes letters to Elvis. But if you want to write a short story or a book, you have to sit your rear end down and write a short story or a book.
There’s only one way to be a writer, and that’s to plant your butt in a chair and write. Write day in and day out, whether uninspired, sick, tired, or hungover. Anotehr way to look at it is that if you want your Muse to help you, she has to know where to find you. If you aren’t sitting down trying to write, why she she bother showing up at all?