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A web page is a great place to centralize information about your book, such as teasers, launch dates, book readings in your area, and author information. When the book is out, you can also include links for people to purchase the book directly from the site.  Your webpage should include the title of the book as its URL, if possible, or be closely related to it and easy to remember. For example, if your book is titled "How to Kiss a Frog," you might want to use the web domain www.kissafrog.com. This makes it easier for people to remember the site address. If you're not familiar with webpage design, a simple template website that provides free blogs should work fine. Check out this helpful wikiHow article for more information on how to build and design your own website. You can also use a Facebook group or product page in addition to, or in place of, a more traditional website. This is probably a good idea if your audience is younger and tech-savvy, and likely to appreciate having updates on your book in their Facebook feed. You should start marketing your book among family and friends before it is even completed, in order to build anticipation before your launch date. The book cover can be used to represent your book every time you post about it online. It should catch the attention of potential readers and give a hint about the genre, contents, and appeal of the book itself.  Since your book is digital only, its book cover is also digital only. In other words, it is just a representative image that you can use online to represent the book itself. It can be any photograph or image. If you have digital media skills and can use tools like Photoshop without much trouble, you might consider doing this yourself. You can also contract the work on websites like Fiverr.com, where it only costs $5. Before the book is available, create a blog or vlog (video blog) entry to post to the book's website. Don't be afraid to brag; now is the time to talk up how awesome this book will be and why people will want to read it. Talk about its unique features and what makes it helpful, interesting, entertaining, or useful.  Don't just write one entry; post about the book frequently. Remember that your website is a reflection of you as a writer, so don't hesitate to make your blog writing engaging, fun, and similar in style to the book itself. Be sure to proof read; having a blog can backfire if your writing is rife with typos and grammatical errors that might make readers question your writing skills! Your book's website should have a feature where individuals can sign up for email updates about the book. Get people to sign up for the email list in advance of the book's release, and that way you will have a ready-made client list when the book is available. Try not to overuse the email list, as most people will not appreciate it and probably view it as spam. Instead, save the email list for important announcements and to drum up business when the book is ready. Once you have a solid presence including a web page, Facebook or other social media presence, book cover, and email list, it’s a good idea to start spreading the word among your family, friends, colleagues and acquaintances.  Try posting on Facebook or Twitter about your book progress and link to your book's Facebook group or website. You can also ask family and friends directly to support you by "sharing" the URLs to your pages on their own social media feeds, or through email or word of mouth. Although it's important to post and talk about your book, be careful not to overdo it; you don't want to drive people away with your zealous promotion.

Summary:
Create a web page for your book. Create an enticing book cover. Blog or vlog about the book. Create an email list. Spread the word in your social circles.