For online resources, please take a look at the websites below, they have precise descriptions and microscopic images:
- http://www.algaebase.org/
- http://www.cyanodb.cz/
I would also recommend the following paper: Preslia 86: 295–335, 2014 (http://www.preslia.cz/P144Komarek.pdf)
Finally, for a systematic classification key, consider the books from J. Komarek and K. Anagnostidis (3 volumes): Cyanoprokaryota. 1. Teil: Chroococcales. Süßwasserflora von Mitteleuropa; Cyanoprokaryota. Oscillatoriales. Süßwasserflora von Mitteleuropa,