The single best resource for learning about Agile testing is Lisa Crispin's excellent book, Agile Testing: A Practical Guide for Testers and Agile Teams.
Other recommended books include:
Agile Testing Condensed: A Brief Introduction by Janet Gregory and Lisa Crispin
More Agile Testing: Learning Journeys for the Whole Team by Janet Gregory and Lisa Crispin
Continuous Delivery by Jez Humble and David Farley
Developer Testing: Building Quality into Software by Alexander Tarlinder
Working Effectively with Legacy Code by Michael Feathers
Articles and Videos
How Programmers and Testers Collaborate on PBIs by Mike Cohn
SelfTestingCode by Martin Fowler
Thoughts on Test Automation in Agile by Rajneesh Namta
Taking Advantage of Agile Testing Practices by Lisa Crispin
3 Pointers to Add Agility to Testing by Torsten Leibrich
Guidelines for Structuring Automated Tests by Sunit Parekh
Tactics for Test Automation of Legacy Code by The DevOps Enterprise Forum
Simplify Your Mobile Testing on iOS and Android by Prateek Kumar Baheti
Gatling: Take Your Performance Tests to the next Level by Rodrigo Tolledo
5 tips for QAs to add more value to their teams by Lina Zubyte
Agile Testing - Nine Principles by Elizabeth Hendrickson
5 Reasons Why Test Automation Can Fail by Torsten Leibrich
The Agile Tester by JoEllen Carter
Future of Test Automation Tools & Infrastructure by Anand Bagmar
Why is test automation the backbone of Continuous Delivery? by Ranjan Sakalley
TDD is the best thing that has happened to software design by Arek Torczuk
Write Better Tests in 5 Steps by Marcos Brizeno
10 ways to ramp up automation test coverage by Haritha Hari
Performance Testing in a Nutshell by Srinivas Murty
Agile Test Automation by James Bach
The QA Role - What Is It Really? by Kenny Cruden
Making a Business Case for Automated Testing by Elena Yatzeck
Solving the Visual Testing Problem by Christoph Burgmer
Faster, better, stronger: Building a high quality product by Finn Lorbeer
Starting Test Automation for a Legacy Project by Vikas Hazrati
The Practical Test Pyramid by Ham Vocke
My Experiments with TDD by Vinay Krishna
Agile Quality Management Model by Yong Kou
Elisabeth Hendrickson Discusses Agile Testing by Jez Humble
Making Test Automation Work in Agile Projects by Lisa Crispin
Agile Testing and Quality Strategies by Scott Ambler
Test Automation - Who Should be Involved? by Kenny Cruden
Agile Performance Testing by Mieke Gevers
How to Decide What to Automate? by Daniel Amorim and Hugo Vares Vieira
Agile testing is not about automation by Ole Lensmar
Managing Technical Debt by Eberhard Wolff and Sven Johann
Tools
FitNesse for acceptance test automation
JUnit for unit test automation
Selenium for UI test automation
Appium for automated testing of native, hybrid and mobile web apps.
SoapUI for automated functional testing of web services
WireMock for stubbing and mocking web services
VirtualBox for creating virtual machines with different configurations
CruiseControl for continuous integration
Cucumber for plain-language test automation
Gauge for business-driven test automation
JMeter for performance test automation
