Email List/Email Service
This is your greatest asset as an author. Should be used to send out a newsletter about upcoming projects, promotions, and book launches. Start building!
An email marketing service is needed to manage your growing list. A couple free* ones to consider are Mailchimp and Mailerlite (*these are free for lists of 1,000 subscribers or less, then you have to start paying).
Website
This gives you a place to put yourself out there and introduce yourself to potential readers. It keeps all your books in one spot and allows you to start building your email list.