A very partial list of blogs and online resources I like.
Only Influencers: https://onlyinfluencers.com/
Marketing Sherpa: https://www.marketingsherpa.com/
SendGrid https://sendgrid.com/blog/
Email on Acid: https://www.emailonacid.com/blog
Hubspot https://blog.hubspot.com (great for all inbound strategies, too),
ReturnPath: https://blog.returnpath.com
Marketo’s blog: https://blog.marketo.com
Really Good Emails: http://reallygoodemails.com/resources
Jason Rodriguez has a nice collection of email resources: https://www.rodriguezcommaj.com/resources
Email Monks (These guys are the best!): http://www.emailmonks.com/blog
EDMdesigner: http://blog.edmdesigner.com
Campaign Monitor: https://www.campaignmonitor.com/blog
Action Rocket: http://labs.actionrocket.co
Wilbert Heinen: Accessible Email Documentation https://github.com/wilbertheinen/accessible-email-documentation
Vision6 (Australia): http://www.vision6.com.au/blog
Books
Guerrilla Marketing Revolution – https://www.amazon.com/Guerrilla-Marketing-Revolution-Persuasion-Unconscious/dp/0749926112
Jason Rodriguez: ‘Modern HTML Email’ and ‘Professional Email Design’ https://www.rodriguezcommaj.com/books