I couldn't find anything about Polish Legions in WWI (in english)
but there is a lot of books about Pilsudski. he was leader of this movement (Legions) and father of this plan (regaining independence by supporting side fighting against enemies and changing sides ;) to beat Russians and Germans later organise up-rises in German emipre and march on east)
(to Less.. Dmowski have done his job perfectly, but I am Pilsudski's fan)
there is quite a lot of books about him on amazon.com. look for positions about 1914-1920. keywords=pilsudski
you should find titles you need and all information about legions)
I know it is wikipedia :/ but all members are not going to buy book ;)