There is no easy way (I did not found any) to download all Gentoo documentation for viewing it offline. Something like "emerge gentoo-howtos" would be very useful.
Has already been discussed: http://bugs.gentoo.org/show_bug.cgi?id=12745 http://bugs.gentoo.org/show_bug.cgi?id=30457 http://bugs.gentoo.org/show_bug.cgi?id=92861 *** This bug has been marked as a duplicate of 30457 ***