Sources of Help for GLScene
Several of the sources of help are for old versions of GLScene and for Delphi rather than Lazarus, but can still be useful if you make allowances for differences such as changed package names. The Lazarus equivalent of Delphi's VCL (Visual Component Library) is LCL, hence the latest Lazarus package names e.g. GLSceneLCL_BASS GLSceneLCL_CgShader and GLSceneLCL_FMOD. These names supersede the shorter originals such as GLS_BASS. Other examples of old package names are glsss_fmodlaz, GLCgx and glscenecgshader.
See below further details of several types of help.
Help File and Source Code
The help file has been generated automatically by PasDoc, so depended on the depth of commenting in the source. Since the help file is hyperlinked, it is much quicker to use than inspecting the source directly. You can see the class hierarchy and inspect separately the types, constants, routines and classes (for which you can drill down to their fields and methods).
A wiki at http://glscene.pbworks.com/ (last edited in 2012) has a reference section with detailed class information accompanied where necessary by images. It has notes and images of the supplied samples and also Tips and Tricks.
Lazarus Wiki page (last modified in May 2014 gives a good introduction to the capabilities of GLScene and shows screenshots. It covers several versions up to 220.127.116.11.
Jan Zizka's 36 page beginner's guide (copyright 2005) refers to Delphi rather than Lazarus and uses some outdated filenames. For example, ode.dll is now ode_single.dll. However, much of the content still applies.
The scribd tutorial entitled Build a basic scene provides clear instructions for Delphi supported by many screenshots. It should also be useful for Lazarus. It links to the SourceForge wiki page Rotate Objects for the next page of the tutorial.