You can ask any speaking author where the majority of their sales come from and most will say from the back of the room (BOR). A great speaker attracts book sales. Many want the rest of the message, more information, more knowledge or more entertainment and quickly gobble up whatever product the author has available.
If you speak, don't miss the opportunity to mention your books. For certain, someone in the room will be interested and want a copy of one or all of them. After all, books provide information we can take home with us and study on our own time. Much more is normally included in a book than a 20-30 minute lecture. BOR is where the sales are.