Complex sentence: Zelazny also contributed to a spin-off work, "The Visual Guide to Castle Amber" (1988) which was a reference work detailing biographies of the Amber characters and a detailed guide to Castle Amber itself.
Split into simpler sentences: Zelazny also contributed to a spin-off work, "The Visual Guide to Castle Amber" (1988). The Guide is a reference work providing biographical detail on the Amber characters and a walk-through guide to Castle Amber itself.