Den 1: Neverwhere (Collects Heavy Metal No. 3-13, 1977ΓÇô1978)
Fantagor Press, 1991, 108 pages (this edition)
Den 2: Muvovum (Collects Heavy Metal No. 54-72, 1981ΓÇô1983)
Fantagor Press, 1991, 103 pages (this edition)
Den 3: Children of Fire (Collects Children of Fire No. 1-3, 1987ΓÇô1988)
Fantagor Press, 1991, 77 pages
Den - Fantastic Adventures, No. 1-10, 1988ΓÇô1989, collected in...
Den 4: Dreams (this torrent contains issues 1-5, not the trade paperback)
Fantagor Press, 1988-1989, 32 pages each
Den 5: Elements (this torrent contains issues 6-10, not the trade paperback)
Fantagor Press, 1989, 32 pages each
DenSaga No. 1-4
Fantagor Press, 1992ΓÇô1994, 30-34 pages each
Den is Written and Illustrated by Richard Corben
Den - Fantastic Adventures and DenSaga contain some contributions from a few other people, as well. Jan Strnad, of course, is one of them.
Den has an extensive and well-written Wiki page that can tell you all you need to know about this character if you're not already familiar with him.
Caveat: You either like Richard Corben's art, or you don't. Unfortunately, for whatever reason, it doesn't scan very well (and that's nearly every scan of every Corben book I've ever seen digitized). The first half of Den 1 looks particularly rough, so if this is your first experience with Den, don't give up until you've reached page 60 or so.
Anyway, here's approximately 730 pages of Richard Corben's Den. Hope you enjoy.