I've been doing some more digging on the 2nd novel, since it was not listed on Amazon, and as soon as I turn something up elsewhere it suddenly appears on Amazon anyway..
The title is 「ジェミニ・ナイヴ 少女たちの遊戯盤」, which translates to something like "Gemini Knives: Girls' playing board", and it will be released on the 29th December 2006 (the same day as the release of the new Gocky Club at Comiket, although Amazon has it down for the 26th) priced at ￥578 and is also listed on the Gakken store along with a description and picture of the cover. For reference, the ISBN is 4059035130.
My copy finally shipped this.. well yesterday morning now actually. Looking at Yesasia and AmJ's listings again now it appears the release date was delayed until the 19th Jan for some reason.
Book arrived today. Only had chance for a flickthrough, but has a couple of new colour illustrations and a character list at the back with some more GMB agents.