The book of solitude allows you to fight with one character rather than four. IIRC it is located in the optional dungeon Sandshroud Ruins.

From what I've read, people seem to say NGS1 is harder than Sigma 2. I've played Sigma 2, and I can say that even if they say Sigma 2 is easier, it still is very difficult (on the higher difficulties). Also, you can replay chapters in Sigma 2, I don't think you could in Sigma 1.