Reading further is not a very easy topic in the area of container virtualization. For systemd-nspawn, it's relatively easy: the man pages are an easy read. Maybe a suggestion that is relevant for systemd-nspawn, Rkt, and even Docker: Red Hat provides on their website a document called The Resource Management Guide (https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/resource_management_guide/) with good information about cgroups.
The documentation for Rkt is not always up to date. Double-check it with their GitHub project: https://github.com/rkt/rkt/. It's the same for Container Linux, check out also their GitHub project on https://github.com/coreos/docs/. There is a book from Matt Bailey about Container Linux: CoreOS in Action, that is a must-have if you are going to play with Container Linux and Rkt: it really provides everything...
