For intro level: Kubernetes Up and Running. (Here's a free version provided by VMware: https://www.vmware.com/content/dam/digitalmarketing/vmware/e...). Will teach you the basic vocabulary and get you well enough oriented to use k8s.

For trying to get to pro level: Programming Kubernetes. This one is focused on writing code for the k8s ecosystem, but it will teach you a lot of the internals.