There were two similar questions, but on military records: Returning German air force record book to family of Walter Schamberger of Sonneberg? and Returning World War I era German dog tags?
My answer given there doesn’t apply here, as were are not talking about military records. I am not aware of any archive that is interested in such membership booklets (except from prominent historical figures).
Locating the family:
Werner is a very common name. You could look for the birth place in the booklet or any information on places of residence in there. Then you might check directories like Das Örtliche to contact possible ancestors (people with that surname) in that area. I consider a match unlikely.
I would scan it, post it to Flickr with the name and some additional information (so it can be found) and keep it as an obscure detail of my family’s history.
Keep in mind, that getting your Nazi party membership booklet back might not be in everybody’s interest as well.
PS: There is also a small, but well curated family history project in Germany that makes scans of family documents available to the public: http://www.dilibra.com/ Maybe they are interested in the scans.