It's never too early to start marketing your book.
Marketing starts even before our books are published.
Book marketing should be a planned process, an art if you will.
As much as we don't want to hear it, book marketing is a huge part of becoming a successful author.
Be knowledgeable in your niche, provide some information free of charge, and share other trustworthy people's free resources whenever possible.
One of the best book marketing tips I can give you is simply building relationships - well that and publishing more books.