Sigil, the best EPUB editor, allows you to create or edit an ebook of any complexity. The programme already includes many useful tools for automating and speeding up the making EPUB process. Once you have mastered them, you will be able to reduce time and improve the quality of your EPUBs.
Search, Replace, Regex
Newcomers to EPUB editing may at first be overwhelmed by the abundance of tags, styles, attributes and other things in the file markup. But in fact there are not an infinite number of tags, and styles are repeated, so they can be typed and mass search-and-replace operations can be performed. Regular expressions were invented to speed up this operation, and if you learn the most basic expression, how to find something in markup or text (.*?) and how to substitute it in the right place (\1), it will cover your basic needs.
Clips
This is a panel for storing and quickly adding code clips to the markup. Using it is very easy: you need to select text or put the cursor in the desired place, and then select and apply something from the list of clips. To the default code clips I added those that I need most often: <blockquote> tag for proper markup of quotes and inserts, tags for empty line, image styles, etc.
Split Marker
If you export a layout from InDesign and forgot to set up splitting the text into XHTML files, you can insert a split marker directly into Sigil (but not by hand, please) and split the EPUB into parts. Or another use case: InDesign can do a lot of things, now it can make illustrations instead of the artist, but it still doesn't know how to put footnotes at the end of the whole book, not after chapters or (never do this) paragraphs. That's where the Split Marker comes in, where you can quickly cut footnotes from chapters so that you can reassemble them at the end of the book into a single XHTML file.
Change Case
If your book editor gives you a text in which the headings of some level are capitalised, but you don't need them in your layout, you can change the case of a word or any text fragment. Once you've mastered Regex and learnt the hint in Clips, you'll be able to make mass case changes.
Removing unused styles and images
I always remove unused CSS styles and images before final validation of an EPUB using the epubcheck utility. Superfluous styles that you cleaned up in the markup itself, but didn't remove from the CSS file, can create you some types of validation errors, and you'll clean that up anyway. And deleting the image files themselves that you removed from the layout during the layout is taking care of your readers, now they'll have to download and store a few megabytes less information on their device, and that's always nice.
As a bonus, I would like to remind you about Sigil's ability to rename XHTML files and images in bulk (for example, to replace special characters and spaces), as well as to open pictures in the image editor and save the result of editing directly into EPUB.
And someday I'll tell you about external plugins and tools I use with Sigil to improve the speed and quality of making EPUB.