[Ripplinger]

I find Sigil's TOC to be very nice and I always use when cleaning up horrible looking ebooks. The only requirement is that the chapters need to have a header tag assigned such as <h1> and not just be a regular <p> font style, and badly formatted books usually are not consistent in those headers. It's the only way Sigil (or any program) has to know what is a chapter and what isn't. Then when you click on "Generate Table of Contents" it will pick up any and all chapters with any <h> tag. Sometimes they're nestled or sometimes a blank line will pick up the <h> tag, just uncheck the ones you don't want to show in the TOC. Then select Tools, TOC, Create HTML TOC and done.

Sometimes the original book is done so badly though you might need to do some editing beyond just checking for <h> tags. I've found some chapters will have a title in them, making a longer chapter name, and others in the same book do not. You'll have to manually correct each chapter to read the same, whether you want to include the longer chapter names or keep it simply to "Chapter xx". That's not a shortcoming of Sigil, it's just a badly formatted book where you need to correct another person's laziness.