Why has my Help Center been reduced to this-'The selected topic is currently unavailable'?

Reset Help center

Most Help Center problems are the result of corrupted Help Center cache or preferences files. The following steps should resolve such problems. Note that these instructions cover multiple versions of Mac OS X: you may have only some of the files or folders cited for deletion.

1. Close or quit Help Center if it is open.

2. Click the red Close button in the upper-left corner of the Help Center window.

3. Trash the following files in your Home > Library > Preferences folder, if they exist:

       com.apple.help.plist
       com.apple.helpd.plist
       com.apple.helpviewer.plist 
   

4. Trash the following folders in your Home > Library > Caches folder, if they exist:

       com.apple.helpd folder
       com.apple.helpviewer folder
   

5. Empty the Trash. If you receive an alert stating that the Trash cannot be emptied "because the item Cache.db is in use," then terminate the helpd process using Activity Monitor. The helpd (help dameon) is a process that provides various services to Help Viewer.

Next Step (if the above doesn't work)

Enable Help Center logging.

1. Quit (⌘ + Q) Help Viewer if it is open.

2. Launch Terminal, which is located in the Macintosh HD > Applications > Utilities folder.

3. At the Terminal prompt, copy this command:

    defaults write com.apple.helpviewer HelpViewerDebugging -bool YES 
   

4. Press Enter.

5. Close Terminal or (⌘ + Q).

6. Launch Help Center by selecting Finder > Help > Help Center. All Help Center actions will be logged in console.log, which you can access via Console. Note that Help Center logging will write a lot to the console.log.

Edit your question and include the relevant outputs of console.log.

Update

Steve, I'm including this as an update because it would have been too long as a comment.

1. Do you have the OS X installer dmg for Snow Leopard or Mountain Lion? If yes, you could use Pacifist to extract files that are necessary for Help Center. (By "clean re-install", do you mean you wiped your computer and reinstalled the OS?)

2. In regards to the console.log, the event.layerX and event.layerY has to do with jQuery for a web browser (Chrome I think?). I know that it is not related to the Help Center in any way. The first few lines:

    Could not find access page in directory /Applications/Debut.app/Contents/Resources/help 21/08/13 5:23:29.343 PM helpd[643]
   

mean that the .help files are missing in your applications (Debut and SketchbookExpress).

This is the best link I can find in regards to Help Center, although it is deprecated and meant for older versions (This is where I got the steps to reset Help Center. I've went through it already but take another look.

Help books are installed in one of two ways:

1. Directly in the Macintosh HD > Library > Documentation > Help folder.
2. Distributed as part of an application's package file and contained within the package of the application for which the help book provides help information.

Check that file path and see if you have help books in Library => Documentation => Help. For instance, I have:

AppleScript.help - 859 KB
HelpCenter.help - 1.6 MB
MacHelp.help - 21.4 MB
VoiceOver.help- 1.4 MB

If you don't have those files, or they are empty or something, you may want to redownload those (This is where Pacifist might come in handy).

As for the second way that help books are installed, it seems that you are missing it for Debut.app and SketchbookExpress.app. I am not sure how you would resolve this, maybe reinstall the application? What happens if you redownload the application, double click and click Show Package Contents and then copy the help folder from the downloaded application to your installed application?