I have now ported my utility HelpHelp, which helps developers and others solve problems with Help books, to Swift 3.2 and built it using Xcode 9. This should ensure that it is fully compatible with both Sierra and High Sierra.
HelpHelp version 1.1 lists available Help books, lets you add a new Help book (for example from a new build on an app which you are developing), and searches existing books. It is available from here: helphelp11
and in Downloads above.