Hedenäs, L. 2003. The European species of the Calliergon–Scorpidium–Drepanocladus complex, including some related or similar species. Meylania 28: 1-116

This article from Meylania looks at European mosses of wetland habitats, and includes a key and detailed descriptions of many of these species which are difficult to identify in the field. It is available as a PDF download.