When you install Houdini, you also install online help that includes documentation and a library of example files which are used to illustrate key tools and techniques. If you choose to not install this material, then you can access the docs (but not the example files) using the links shown below.
The Hqueue documentation helps you set up a job management system that distributes, monitors and manages tasks across a collection of computing nodes, or client machines.
The HDK documentation is for the Houdini Developer’s Kit which is a comprehensive set of C++ libraries that give third party developers access to the same API used by developers at SideFX.
The Houdini Engine Documentation gives you information about Houdini's compact Engine API which extracts Houdini's core technologies into a procedural engine for film and game studios to integrate into other applications. With this documentation you can create a plug-in for any number of different DCC apps.