One of the things that happens as markets grow in size is market segmentation. Back twenty years ago when the homeschool movement was still in its infancy there was a very small market for books on homeschooling. Even ten years ago books like "Getting Started on Home Learning" by Rebecca Rupp were fairly general, because the market was still fairly small.
Now with probably over two million children being homeschooled, and a fast growing market, market segmentation is evolving for books on homeschooling. Amazon has hundreds of books on homeschooling. There are books focusing on various aspects of homeschooling. There are books for teaching specific ages of children at home. There are books about homeschooling children in various religious settings. Isabel Lyman's book "The Homeschooling Revolution" focuses on just "What is homeschooling?" The book doesn't try to teach people how to homeschool. All of this is evidence of an increasing market for books on homeschooling, and the increasing development of homeschooling books for targeted markets.
I believe there is a market for a book which focuses on why people homeschool, and why the reader should homeschool. The book could be fairly small, say around 150 pages. My working title is "101 Reasons to Homeschool." There would be chapters on academics, religious, social, fun, and miscellaneous reasons.
The target audience would be
1) Those who are considering homeschooling.
2) Those who don't plan to homeschool, but want to understand it better. This group would include family, neighbors, and so on.