Outside Unreal related tutorials there are a lot of resources that can help you grow as a UI designer and creator.
If you're serious about design, it's good to know about typographic terms, theory and some practical guidelines.
- Detail in Typography by Jost Hochuli — If I had to recommend a single typography book, it would be this. It's concise but detailed. Covers the basics but doesn't skip more nuanced topics. Has a great mix of theory and practical application.
The Elements of Typographic Style by Robert Bringhurst — More of a reference book than Detail in Typography, I still found this fairly useful.