Web sites
Scrum Alliance - scrumalliance.org - A non-profit organization that provides Scrum certifications and lots of useful information.
Scrum.org - scrum.org - Scrum.org provides comprehensive training, assessments and certifications to improve the profession of software delivery.
Agile Alliance - agilealliance.org - a nonprofit organization providing a global set of Agile resources, events, and communities.
ICAgile - icagile.com - a community-driven Agile certification and accreditation body.
Scaled Agile - scaledagile.com - information and certifications for the Scaled Agile Framework (SAFe)
LeSS (Large Scale Scrum) - less.works - home of the LeSS framework - devoted to applying Agile and Scrum to very large and multi-site product development.
Retromat - retromat.org - a great resource for ideas on running retrospectives
Tasty Cupcakes - tastycupcakes.org - a community-run site dedicated to Agile games, techniques, and approaches.
Gamestorming - gamestorming.com - a set of co-creation tools used by innovators around the world
Fun Retrospectives - funretrospectives.com - Activities and ideas for making agile retrospectives more engaging
Books
Coaching Agile Teams by Lyssa Adkins
Essential Scrum by Ken Rubin
Agile Project Management with Scrum by Ken Schwaber
User Stories Applied by Mike Cohn
Agile Retrospectives: Making Good Teams Great by Esther Derby, Diana Larsen, and Ken Schwaber
Unlocking Agility: An Insider's Guide to Agile Enterprise Transformation by Jorgen Hesselberg
Agile Project Management for Dummies by Mark C. Layton
Collaboration Explained by Jean Tabaka
User Story Mapping: Discover the Whole Story, Build the Right Product by Jeff Patton
Crucial Conversations by Kerry Patterson, Joseph Grenny, Ron McMillan, Al Switzler
The DevOps Handbook: How to Create World-Class Agility, Reliability, and Security in Technology Organizations by Gene Kim, Jez Humble, Patrick Debois, and John Willis
Extreme Programming Explained: Embrace Change by Kent Beck