Octave is fully documented by a comprehensive 375 page manual (a Český (Czech) Version and a 日本語 (Japanese) Version are available). The version available here has been converted to HTML directly from the Texinfo source files that are distributed along with every copy of the Octave source code. You can also use those same source files to create a printed copy of the manual, or a copy suitable for viewing with GNU Info readers (within GNU Emacs, using the standalone Info reader, etc.). The complete text of the manual is also available at the Octave prompt using the command help -i (this command invokes the GNU Info browser for you).
A printed version of the Octave manual is also now available and may be ordered from http://www.network-theory.co.uk/octave/manual/. Any money raised from the sale of this book will support the development of free software. For each copy sold $1 will be donated to the GNU Octave Development Fund.
If your questions are not answered by the manual, or by this web site, there is also the FAQ, a list of frequently asked questions with answers. The copy of the FAQ available here is generally more current than the version distributed with Octave releases.
Finally, if even the FAQ doesn't provide the answer you are looking for, the help-octave mailing list exists for the discussion of matters related to using and installing Octave.