Books

I'm not sure if anyone in this community is doing this type of thing, but I know that there is a volunteer community of ebook editors. I've come across some books in my field that have very poor ebooks and I'd like to try editing them at least for my own use and, where appropriate, public consumption. I've tried using the built in Calibre Ebook editor which is functional but a bit of a slog. Is there dedicated software (preferably FOSS) That is really great for editing ebooks? Thanks in advance for any recommendations!