Snort Users Manual:

http://manual-snort-org.s3-website-us-east-1.amazonaws.com/

Snort Rule Writing Manual:

http://manual-snort-org.s3-website-us-east-1.amazonaws.com/node27.html

Infosec Institute Snort Rule Writing Overview:

http://resources.infosecinstitute.com/snort-rules-workshop-part-one/#gref

Emerging Threats Snort Rules:

https://rules.emergingthreats.net/open/snort-2.9.0/rules/

Snort Community and Blog Network:

https://snort.org/community

Security Onion Google Group:

https://groups.google.com/forum/#!forum/security-onion