Book [online, free]: "The Scrum Guide" by Jeff Sutherland and Ken Schwaber
Book [paid]: "Essential Scrum" by Kenneth Rubin
Organizations:
The Agile Alliance - The earliest organization, first chaired by one of the original authors of the Agile Manifesto: Ken Schwaber. Since 2001.
The Scrum Alliance - A training and certification organization and CoP, also founded by Ken Schwaber. Since 2002.
Scrum, Inc. - A training and certification organization, founded by another of the original authors of the Agile Manisfesto, Jeff Sutherland. Since 2006.
Scrum.org - A training and certification organization, also founded by Ken Schwaber, dedicated to further refinement of professional agile practices. Since 2009.
Scaled Agile Framework (SAFe) - A methodology, training, and certification organization and CoP designed to support and scale the team-oriented agile framework up through the enterprise levels by synchronizing up to 100s of teams to help them deliver business value. Founded by Dean Leffingwell. Since 2011.
There are many certifications for Scrum Masters, Product Owners, and the roles within the various forms of enterprise (scaled) agile. The gravitas of each is a topic of debate in agile Communities of Practice (CoPs) because certifying bodies differently emphasize and prioritize among the essential responsibilities within the roles and vary in their requirements for book knowledge, in-person training, and lived experience. More and more, organizations are choosing to invest in developing internal agile coaches and trainers (validated by recognized agile organizations, such as the Scrum Alliance and SAFe) who then train the rest of the organization. Internal, professional agilists can help an organization scale its implementation by reducing the cost of training and creating consistency and lending relevance to the organization's specific situations.
There are oodles of places to get book knowledge (including this website). But until you get to try out a role and get to see the agile mindset modeled, it's hard to know how to start. Experiential learning is highly recommended and available through the organizations listed in General Resources, above.