I’ve discussed this with a number of book editors and a few agents. Without exception, they urge writers to learn the craft and usually add, "The best way to learn is to start with shorter pieces."
The first question an acquisitions book editor often asks a prospective new writer is this: "What have you published?"
If you answer, "Nothing," you've already given your material a negative impression. You can overcome it, of course, but it's better to be able to say, "I've published 12 print articles and had 10 stories in ezines."
Start by writing short pieces is good advice.