Many consider Don Norman, the author of The Design of Everyday Things, father of modern UX design. If you want to absorb the basic principles of usability, you should read it.
Does it primarily focus on usability or also on beauty? I am slowly finding out that often less is more when it comes to UX design, also in the way things look, and would love to find out more of these principles that can vastly improve the looks of a product (e.g. what colors to use, shapes, etc.)
This book is about UX. That is not how "it looks" but how "it feels". I think for UI, as mentioned in other comments you might want to check out https://refactoringui.com/
Does it primarily focus on usability or also on beauty? I am slowly finding out that often less is more when it comes to UX design, also in the way things look, and would love to find out more of these principles that can vastly improve the looks of a product (e.g. what colors to use, shapes, etc.)
Thanks again!