I've tested on kde-misc/krusader-2.3.0_beta1 and on kde-misc/krusader-2.4.0_beta1. Emerging with USE=handbook fails if following packages are not installed: =app-text/docbook-xml-dtd-4.2-r2 docbook-xsl-stylesheets I'm not a really guru so may be there are more packages that krusader is missing. But theese two are ones, which are not RDEPENDed by any package on my system and are unmerged when cleaning world.
IUSE="handbook" replaced with KDE_HANDBOOK="optional". Thanks for reporting.