Redis University is free. The quality of materials is superb. I fully prepared for Redis Certification exam using only Redis University materials / Official redis docs. https://redis.io/university/
In case you are interested in internals, I found code crafter to be very useful for learning.
Are you interested in Redis specifically or a general understanding of how to think about key-value DBs?
antirez, founder of redis, has covered some advanced topics in his blog http://antirez.com/latest/0