Most publications deal with temporary work agencies. There is only general discussion regarding placement agencies. I am looking for studies that include more detailed, especially regarding regulations that are used in different countries.
We did a study on public employment agencies, it was an evaluation of the reform of the German institution in the framework of the Hartz III reform. We cite a lot of previous work which is also related to private employment agencies. Please have a look at our paper "The Employment Effect of Reforming a Public Employment Agency" which is available here in research gate. The published version (European Economic Review) is at http://www.sciencedirect.com/science/article/pii/S0014292115001245