[ozzythecat]

“Backend” can refer to a lot of different things. What specific kinds of problems are you wanting to learn more about?

Without more details, you’ll probably get folks recommending whatever language they prefer - to do whatever it is they’re doing on the backend. It’s not necessarily what you care about.

And to be a “senior” developer implies you’re wanting to make use of this at some company.

Again, what is the expectation of the role? Writing services in Go or Java to handle FANG scale traffic is different than building a backend workflow that migrates data from one system into another at some recurring cadence.

One book that I think has a lot of value for different backend problems is “Designing Data Intensive Applications” by Martin Kleppman. It’ll help you build a foundation of the different varieties of architecture and problem spaces in this area and a toolkit on how to think about them.