The topic is so huge, I do not exactly understand what is your idea or concrete topic (overeducation, unemployment experiences, mismatch...). I recommend you to read some exploratory papers that could help you to define more your area of research:

- "Higher Education and Graduate Employment in Spain" (2000), by Jose-Gines Mora, Jose Garcia-Montalvo and Adela Garcia-Aracil in European Journal of Education. 

-"The Experience of Unemployment for University Graduates: An Exploratory Study" (1990) by WILLIAM A. BORGEN and WENDY E. HATCH, Journal of Employment Counseling.

- "Entrepreneurship and market size. The case of young college graduates in Italy" (2010), by Sabrina Di Addarioa and Daniela Vurib, in Labour Economics. 

- "Overeducation in the graduate labour market: A quantile regression approach, (2007), by  Séamus McGuinness and Jessica Bennett, in Economics of Education Review. 

-"The class of 1981: the effects of early career unemployment on subsequent unemployment experiences" (2003) by Simon Burgess, Carol Propper, Hedley Rees and Arran Shearer in Labour Economics. 

On this link you can find very intersting papers on the subject of your interest:

 http://www.careers.ac.rs/survey.html 

Maybe you'll find something at the INCHER:

http://www.uni-kassel.de/einrichtungen/en/incher/home.html

or at the national Institute of Employment research (IAB)

http://www.iab.de/en/iab-aktuell.aspx

But i think graduate unemployment is actualy not a big issue in germany.

Thank you for your answers. I will be sure to follow up on these leads.