Hi,
'Die Pistole 08' by Goertz (last edition with blue cover) does about 30 - 40 Euros over here. There are still loads of them available (see Amazon and German Ebay).
There is also a good supply of new 'The Dutch Luger' books from Martens & De Vries. They will do some 40 - 60 Euros at the moment.
A good place to learn the value of books is the
www.abebooks.com website. Also ebay and amazon searches can put you in the right direction.
Having a signed copy is nice, but not spectacular, because as Ed said the authors are only known in collector's circles. That said, I would add a percentage to the value of a signed book, say some 10%.
Numbered (limited) editions have a value of their own.