What are some further readings in Econometrics you recommend?
I might be wrong but from what you write, it seems you've been given a "classical" introduction to econometrics: You've covered IVs and Diff-in-diff but apparently only in passing, and causal inference does not look like it was the core of the classes you've taken.

If that's correct, then I would recommend reading:


Mastering 'Metrics (https://www.amazon.com/Mastering-Metrics-Path-Cause-Effect/dp/0691152845),
Mostly Harmless Econometrics (https://www.mostlyharmlesseconometrics.com/)


before reading anything else (e.g., Wooldridge or Greene, as referenced in E.Sommer's answer).