I highly recommend https://refactoringui.com/book/. It's not SaaS specific but it very much approaches design from a developer's perspective which has helped me immensely in understanding and implementing small tweaks that make huge improvements.

I also recommend watching his YouTube videos. They can get long but it's nice to see the thought process behind a visual refresh/refactor from beginning to end. https://www.youtube.com/channel/UCxqiDtkXtOCNJdckODHk9YA/vid...