Nexus is the exoskeleton of scaled Scrum. It drives to the heart of the scaling issue - continually identifying and removing dependencies created by increased complexity. It builds on the existing Scrum framework and values. The result is an effective development group of up to 100 people using best industry practices. For larger initiatives, creating product families or interoperating functional units, we create Nexus +, a unification of more than one Nexus.
Nexus is a another way to scale Scrum to larger number of people (more than 9), besides LeSS (Large Scale Scrum), SoS (Scrum of Scrum), SAFe (Scaled Agile Framework), among which Nexus is one of the most simple. The guide itself is also short (11 pages), simple and easy to understand, but you must have solid foundation of Scrum first, preferably from Scrum Guide and other agile values and principles.
Description of a little scaling framework on top of the scrum framework. Never used in practice (nor know of any organization using it), so can't judge its usefulness.
It's a short read and not as densely packed as the scrum guide.
(Read this to broaden my scrum knowledge beyond regular scrum and the SAFe framework that is all the rage in these regions these days).