Apart from the official, very comprehensive documentation, there is an awesome (it's even named that way - Awesome Docker) list of Docker resources available on GitHub https://github.com/veggiemonk/awesome-docker.
The list of resources has a good structure and begins with the basics like tutorials explaining what Docker actually is and how to install it on various platforms. The list is divided into main areas, which, among others, include:
Useful articles: This contains a list of articles related to various subjects. Docker networking will list articles related to Docker's networking internals. The Image optimizations section will tell you how to create the smallest possible images and how to create Dockerfiles with the image size in mind. The security section lists resources related to security best practices, auditing, improving container security, and so on. Performance related articles will let you improve the performance of the containers. The Good Tips section is what I found the most useful. It contains a list of tips and tricks, coming from the people who use it in real life - these are very valuable.
Tools: This contains references to a lot of developer and devop tools that are related to Docker. It will include tools for doing continuous integration or continuous delivery, monitoring, and logging; the list is endless, you can find something useful for sure.
Slides/Videos: This holds a list of additional resources available as slides for presentations and a list of videos, webinars, and so on.
People, twitter accounts and communities: And last but not least, a catalogue of people important in the Docker world. You will be able to find a list of blogs, and twitter accounts worth following and also a list of Docker related communities you can join.
The Awesome Docker list is updated very often and has a lot of contributors. You will always find a lot of interesting material here. For example, you will find links to the most up-to-date Docker Tips articles and blogs, a list of useful Docker third-party developer tools and recipes for integration with many continuous integration/continuous delivery providers. If you need to host your Docker container, you can get the information about almost all available Docker hosting providers. It's really hard to imagine that you wouldn't be able to find something you are looking for if it's related to Docker. There are even links for running Docker on Raspberry Pi. My advice is to clone the repository to be able to update in the future to find more interesting links.
If you find something interesting you would like to share with others, you are more than welcome to contribute to the Awesome-Docker-list; the authors appreciate anyone who would like to add something to the list.
Another resource I would like to mention is the Docker Jumpstart, by Andrew Odewahn. It's also available on the GitHub in the form of mark-up documentation. You can get it at the https://github.com/odewahn/docker-jumpstart/ address. It's very concise and complete, good for refreshing essential Docker skills.