One of OS X’s most vexing features is that, when you summon Help in almost any application, the Help Viewer window floats on top of every other open window. Even if you switch from the Help Viewer to another program, the Viewer window stays on top.
Fortunately, there is a fix: open Terminal (/Applications/Utilities) and at the command line type defaults write com.apple.helpviewer DevMode -bool true, then press Return. Now, the next time you open the Help Viewer, it will no longer float on top of your other windows. To return to the default behaviour, enter defaults delete com.apple.helpviewer DevMode.