What are some books or courses which had high ROI on your career?

my Safari books (oreilly.com) online subscription! https://www.oreilly.com/online-learning/pricing.html

Visual Quickstart Guide for SQL.

I still give it to my new data analyst interns, so many are excel jockeys and it's the most consumable intro and reference to sql I know for a someone intimidated by SQL. Combine that with coding horrors join diagrams from 2007 and you've got even non technical people off to the races even if they aren't particularly self guided learners.

I wish I had an equivalent for R though, still don't know how to ease people into that.

The book Impro - it's a fantastic book written by an Improv coach, the first chapter on Status was one of those lightbulb moments for me, so many interactions I had everyday made so much more sense. Reading the first chapter is a better use of an hour than anything I can think of off-hand.

https://www.amazon.com/Impro-Improvisation-Theatre-Keith-Joh...

Learning How To Learn (available on Coursera here [0]) and its companion book[1]. That course seriously improved how I learn new topics and has paid dividends many times over since then.

[0] https://www.coursera.org/learn/learning-how-to-learn

[1] https://www.amazon.com/Learning-How-Learn-Spending-Studying/...

Vladimir Khorikovs Pragmatic Unit Testing course on Pluralsight. Massive game changer for me.

How to make friends and influence people by Dale Carnegie - Learning how to talk to people in a way that make them feel validated has been the most important lesson I've learned for working effectively in teams.