Quick win : https://www.udemy.com/course/raspberry-pi-full-stack-raspbian/ . I mentioned doing this during an interview and was surprised by how well it was received. I didn't even put it on my CV because I thought it was too simple.
Not so quick win: this book .
We used this book in our embedded linux uni course. It gave me solid basics and was fun to read/follow.
These two things were all I had related to linux and were enough to land a job.