I am a senior dev in a startup, and have a motivated part-time tester that wants to build a career in QA. She does manual testing and writes Selenium tests for a while now, but has no background in QA other than that.

What books or articles or blogs should she read, what exercises might I give her, how can I challenge her so that she gets the theoretical foundation, the tools and techniques, the mindset to really excel in her job?