In the Netherlands we use several papers on polychaetes, often for each genus/species-complex a specific paper. But we start by using two guides, the one by Hartmann-Schroder (1996) on the German Polychaetes, but this is out-dated but still of use as a start. Next to this book, we prefer to use the PhD thesis (3 volumes, only as PDF) by Joao Gill (2011) on the Polychaetes of Europe. It is more complete and deals with all European species at that time, but lacks a lot of figures. To understand his thesis we often refer to the figures in the numerous papers on this genus/family. We know there are books on Spanish Polychaetes but we don't have those (lots of figures but in Spanish :-(
HARTMANN-SCHRÖDER, G. 1996. Annelida, Borstenwürmer, Polychaeta. Die Tierwelt Deutschlands und der angrenzenden Meeresteile nach ihren Merkmalen und nach ihrer Lebensweise. Begründet 1925 von Friedrich Dahl. 58 Teil, Gustav Fischer Verlag, Jena, 2nd revised edition, 648 pp.
GILL, J. 2011. The European Fauna of Annelida Polychaeta (Vol. I-III).PHD-thesis, University of Lisbon.