Regardless what the topic of your book is you can be sure there are at least one or two forums for people who are interested in that topic. They’re easy to find. Just do a Google search for “keyword forum” replacing the word keyword with an appropriate keyword pertaining to the topic of your book. There may be several good keywords for your topic. Try them all.
When you find forums, join them and start reading some of the threads being posted by the members. Don’t be in a hurry to post or comment right away. Wait until you have a general feel for the atmosphere in the forum and see how the group interacts with one another. It won’t take long to get an idea of the lay of the land. No two forums will be exactly alike.
Eventually you will come across a question or thread that fits your scope of expertise. Start making positive comments that are helpful to the understanding of the topic involved. Don’t be too quick to start plugging your book. Let the subject come up naturally and reply modestly.Once the word is out that you’re a published author the interest of others will expand rapidly.
You must decide in advance whether you wish to give away copies of your book (a PDF version would be ideal for this), sell them a hard copy directly (having a book website to send them to would work well), or simply send them to your link on Amazon and let them decide between a paperback or Kindle edition. Your choice in this matter will likely be guided by the type of forum. If it’s a group of professionals in your field for instance, you might offer to let them give your book to their clients for a reduced price.
Contribute to forums freely and often once the ice is broken. Give advice and reach out. Offer to help others. Be the expert others turn to. Put a link to your blog or website in your signature line. When you have a book contract and/or a book title, add the title to your signature line. Remember every copy of your book that gets into the hands of interested parties is a potential sales tool. You never know who will read it, find it helpful, and turn to you for your expertise. Your book will do most of the selling for you.