I'm assuming you mean the Underdark book. I fso, then it is a Forgotten Realms product. Any books that have/has/will come out as a part of the Forgotten Realms product line, does not and ever will have any Open Content.
However, that should not stymie a creative mind. Read it, understand it, and then extrapolate what is there. You can take any one of the feats or other material, reword it enough (60%, I believe) to call it original, and then publish it.
"I'll swallow your soul," Deadite qoute
"Only if I don't kill you with my +4 Keen Chain Saw first!" - Me