To convert a FrameMaker book to a single HTML output file instead of splitting the chapters into smaller topics, you must start with a single FrameMaker file. Create a new FrameMaker file, and import into the new file, by reference, each of the chapter files, in the same order they appear in the book. Each chapter file becomes a text inset in the new file. You never edit this file again; just update it after you make changes in your FrameMaker book, then use Mif2Go to export the new file to HTML.
For HTML, the best practice is to reduce the size of each page to one topic, preferably a topic that does not require any scrolling to see the whole page. When people scan for information, they might go from page to page without looking past the first screen of each.