Apache NiFi User Guide - A fairly extensive guide that is often used more as a Reference Guide, as it has pretty lengthy discussions of all of the different components that comprise the application. This guide is written with the NiFi Operator as its audience. It provides information on each of the different components available in NiFi and explains how to use the different features provided by the application.

Administration Guide - A guide for setting up and administering Apache NiFi for production environments. This guide provides information about the different system-level settings, such as setting up clusters of NiFi and securing access to the web UI and data.

Expression Language Guide - A far more exhaustive guide for understanding the Expression Language than is provided above. This guide is the definitive documentation for the NiFi Expression Language. It provides an introduction to the EL and an explanation of each function, its arguments, and return types as well as providing examples.

Developer's Guide - While not an exhaustive guide to All Things NiFi Development, this guide does provide a comprehensive overview of the different API's available and how they should be used. In addition, it provides Best Practices for developing NiFi components and common Processor idioms to help aid in understanding the logic behind many of the existing NiFi components.