I will occasionally go into the book editor in Calibre for a few simple fixes, if there’s no easy alternative and the problem bugs me enough. It’s hard to stop once you’ve started, though - I’ll just tweak this, and that needs a capital C, and I might as well space that out a little better… etc. etc. So I try not to, whenever possible. But I do aspire to one day picking a public domain text I love and building a comprehensive, well-constructed free version.
The chief value of the Calibre editor, for me, is being able to remove unnecessarily large images, often shrinking the file down to a small percentage of the original size.
