My circumstances required me to; I worked as a software engineer for a startup without system administrators. So I just solved any issue I encountered while deploying / running production.
Ansible is great; easy to learn and powerful. I recommend the book "Ansible Up & Running".
Learn Linux thoroughly. I recommend the book "How Linux works". You want to know how you can do basic administration such as managing users, setting up syslog, registering system services (systemd/init.d) etc.
Also, learn how ssh works, how you use keys, how ssh config file works, agent forwarding, etc. One of the last chapters/appendixes in the Ansible book goes into detail.
Learn about monitoring, for example using the book "The art of monitoring". Graphite + Grafana works great for me. There are many choices, don't get into the rabbit hole of trying to compare them all. Just set up end to end monitoring and alerting. We use Seyren for alerting which is simple and nice.
I also like the book "Site Reliability Engineering" by some Google engineers. Though this is more advanced stuff, for mature organizations that have the basics covered.