Improving product quality is a long and winding road. Is not something you acheive immediately and there are multiple factors affecting it. Vision, market knowledge, product definition among others. And of course software quality.
The “Art of software testing” has evolved a lot since Glenford Myers published this classical book in 1979. XP made a very strong contribution to improve software quality, promoting a number of very valuable engineering practices.
Martin Fowler published a “Software Testing Guide” including articles and publications about TDD, Self Testing Code, test types and test classification. Is worth to read it here.