(Another question frequently asked)
You should start marketing your book the day you’re sure you’re going to write it. Your website is a major part of your marketing.
Consider how movies are marketed. A year in advance, sometimes more, teasers start to come out. A website goes up with trivia, bits and pieces.
As the date approaches, the teasers turn into trailers, longer more detailed snippets to suck you in and build excitement, buzz.
Just before the launch is when the big blitz happens, but it only matters because real fans have been talking about it since the announcement a year ago.
You’re also right in calling it an author website. Create fans of you, not fans of your book. Sure, let them know all about the book, but if they love the book, you’ll have to re-sell them on the next book, and the next, and the next. If they love you, all you have to do is announce the next book, not sell it.
Your site should talk about you. About the book. About what you’re writing, and why. It should have a realistic timeline. Don’t lie to your fans. They’ll abandon you.
Your site should show potential fans why on earth they should care about you, someone they’ve never heard of, and your book, which doesn’t even exist yet.
If your site looks like just another wannabe writer slapping together one more [insert hot genre here] kind of book, who cares?
But if you have a compelling story about you, about your need to write, about how you’ve already got a powerful outline for a mashup of The Fall of Troy and The Great Gatsby and you can back up your promises, folks will care. The right folks will care.
Assume, from Day 1, that you’ll have fans and non-fans. As we learn in Charlie the Unicorn, shun the non-believers. A thousand true fans is worth more than a million vaguely interested folks. Write for your fans, both in your book and on your website. And if it cheeses off your non-fans and they go away, that’s more Vienna sausages and Ritz crackers for the rest of us at your launch party.