Somewhere between High Sierra 10.13 and 10.13.4, Apple has made major changes to the Help system. Unfortunately, these include radical changes in the undocumented file used by the Help system in ~/Library/Caches/com.apple.helpd/HelpCache.plist.
Those changes not only break my utility HelpHelp, but the information now accessible in those and other Help system files no longer contains that on which HelpHelp relies. In other words, HelpHelp not only doesn’t work any more in the current version of High Sierra, but it will never work in it and subsequent versions.
I am very sorry about this, but unless I can think of an alternative source of the information on which HelpHelp relies, this is not fixable.
If you wish to debug and test your own Help books, I can only recommend that you do so in Sierra 10.12.6, where HelpHelp still works as advertised.