For anyone just starting to explore Dynamics 365 for Operations Retail, here are the resources that were most useful early on.
1. The Retail landing page on docs.microsoft.com
Retail SDK overview — the official starting point for Retail SDK documentation.
2. The Microsoft Dynamics Learning Portal (DLP)
If you're an independent consultant, you can still get access — register through the portal directly.
Update: this is also open to consultants working with end users, not just partners — thanks to Jay Patel for flagging that.
3. dynamicsnotes.com
A very helpful blog by Andreas Hofmann from Microsoft: dynamicsnotes.com
4. Kurt Hatlevik's blog
An important consolidation of Retail and D365FO resources, from Kurt Hatlevik: