I mostly use: Die Haplopoda und Cladocera Mitteleuropas von Dietrich Flößner. But I have some more literature about water fleas. I am no expert on water fleas but I know that it is a very diffecult group for identification so I am carefull in this matter.
Identification of water fleas is difficult for some reasons:
1 Most keys are based on adult females. Very often a water flea under the microscope is not adult and sometimes it is a male.
2 A female with an ephippium (result of gamogenetic reproduction) looks rather different.
3 Cyclomorfism occurs, caused by ecological circomstances. For instance in the Daphnia group.
4 Local forms occur, the same species in an other place may have a different form.
5 To make it still more difficult there is interspecies hybridisation which leads to still more polymorfism. In the Daphnia group this happens often.
For all this reasons it is not easy to identify water fleas, in particular the genus Daphnia.
Nowadays there is much informationon Internet but a lot of used names are false.
This are reasons why I am careful before I give a definite name to a water flea.