There are a wide variety of eBooks that are publicly available on the web- unfortunately many of them have crap formatting (or misspellings, rantogetherwords or missing punctuation). Recently I discovered Sigil, a freeware EPUB editor.
It appears that an EPUB is a file container that can house xml documents, images and other items I have not explored (fonts, cascading style sheets, etc.).
Using Sigil and an EPUB that have a title cover, the cover.jpeg (note it is usually jpeg, not jpg) can be readily be replaced with an updated version.
Sigil allows table of contents to be generated from the header markup data (h1, h2, h3, etc.); this is very useful for removing extra blank pages and regenerating a full table of contents.
Further searching found I can insert images into any xml document. So- just for example- if I had a copy of one of the Wheel of Time books and I felt like adding in the chapter icons, I could download the icons, resize them to a consistent size that would fit on an eReader (100x100 or 100x200, depending on the original size) and then insert and verify the proper icons for each chapter.
After the formatting is completed and saved as an EPUB, I can re-open it in Calibe, verify the formatting looks good and convert it to a kindle format (MOBI or AZW) to read on my Kindle screen.