While maintaining the no-excess-verbage style of the previous version of the Octave manual, the Version 3 manual is greatly expanded and much more useful. For example
the chapter on statistical computations has been expanded approximately ten-fold.
Many, many new features are described, such as the more-Matlab-compatible graphics commands and the Packages system for expanding Octave's capabilities.
Unless you're opposed to books on principle (an online version of the manual is available), I would highly recommend purchasing a copy of the Version 3 manual -- it comes at a bargain price, part of which helps fund Octave R&D.