Quick Google found this suggestion -
http://www.osalt.com/publisher
It appears that Scribus is their recommended DTP package, with OO Draw fulfilling some needs.
UNIX DTP used to be done by commercial applications Interleaf & Framemaker, 1 don't know whether they support Linux (and if they do they're probably not cheap), but Framemaker is now owned by Adobe, hmmm.
Ubuntu have quite a nice little equivalents for Windows users on their site, if you haven't seen it. They might have other suggestions.
I should add there's a vast number of Document production systems, which may or may not be suitable and meet your needs. I'm presuming you want a GUI app style DTP package, and not a more general system. I vaguely remember O'Reilley the publishers discussing Open Source DTP, a search on their site might find some interesting info.