[throwaway81523]

Security mindset: read the book Security Engineering (it is online), less for specific technical info than for the many war stories etc. which will help you guard against vulnerabilities and unforeseen consequences.

Basics of cryptography: there are many dumb errors to avoid.

Antirez's general advice about "10x programmers" is good: http://antirez.com/news/112

Thorough (not just basic) knowledge of SQL, if you don't count that as a language. The sqlite.org "technical and design documents" about sqlite's virtual machine and its query planner are well worth reading, and apply to other databases as well. ORM's are less important than SQL, and are usually language specific as someone mentioned.

Reasonable clue about socket programming, even if you're doing everything with libraries that wrap the details.

Comfort using debugging and profiling tools.

Lots of other stuff, I'm sure.

[comprev]

This book? https://www.cl.cam.ac.uk/~rja14/book.html

[throwaway81523]

Added: Oh nice, there is a third edition (2020) now. I only knew about the 1st and 2nd editions. The 2nd ed is completely online but the 3rd only has sample chapters online: OTOH, the 3rd is in all likelihood an updated/expanded version of the 2nd. So you could read the 2nd and decide about buying the 3rd.

I'd say it's a book for inspirational bedtime reading, rather than careful study or reference. But it's great in that way. Security is about mindset more than anything else, and the book puts you right into it.

[throwaway81523]

Yes. It is a little bit old by now, but it is a great book.