I noticed this in Builder, but it applies equally to DevHelp in the main (which Builder just calls on).
Both the gtkmm and glibmm documentation sets are shown as options in DevHelp, but they're unusable. DevHelp gives an "Error opening the requested link" on the 1st try to open any document in either tree, and is silent after that. Other docs work OK.
/usr/share/devhelp/books contains directories for both projects with .devhelp2 files therein, which seem to point at the correct paths (albeit via symlinks, but that makes no difference) - but neither are usable. I tried changing the path to avoid the symlink, and it made no difference. The files are present at the mentioned location and can be browsed using a normal web browser, just not DevHelp.
Possibly relevant links...
