This manual written by Jochen Martens is a very important reference for European Opiliones and brings fully illustrated characterizations of families, genera and species not only from Germany, but also from neighbor countries. Unfortunately, since many years it is no longer found for purchase.
Martens, J. (1978b) Spinnentiere, Arachnida: Weberknechte, Opiliones. Die Tierwelt Deutschlands. Vol. 64. G. Fischer Verlag, Jena, 464 pp.
Digitalized version in OP.