System design

https://github.com/donnemartin/system-design-primer

Examples of existing systems

Becoming a software architect

Documentation

https://www.divio.com/blog/documentation/

Distributed systems

https://aws.amazon.com/builders-library/challenges-with-distributed-systems/

Whitepapers:

Branch prediction

https://stackoverflow.com/questions/11227809/why-is-processing-a-sorted-array-faster-than-processing-an-unsorted-array

Math for programmers

<3 https://jeremykun.com/primers/

Other

Silly project ideas for learning a language

Career and interviewing

https://80000hours.org/career-decision/article/

https://www.uop.edu.jo/download/research/members/495_1887_llll.pdf (specific to Google but sort of generally relevant)

General interview resources and sample interview questions