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...

I'd second that, I bought it a while back and it's good.

For general design resources, Hack Design has curated a really good collection of introductory design lessons: https://hackdesign.org/

For specific design questions, I like to search the UX Stack Exchange (https://ux.stackexchange.com/) whenever I'm unsure of a UI design. For example, if I'm designing a pagination component or error messages, there are probably relevant questions and answers related to best practices.

I would also recommend familiarizing yourself with the human-centered design process - nothing beats hands-on user research and prototyping. Identify which parts of your interface are causing the most friction, why, and what users are actually trying to achieve.

A lot of UI can be improved with basic visual design concepts - typography, color, spacing, visual hierarchy, composition.

You can use this tool to https://www.visualeyes.design/ to get attention maps of a design.