The cashbook endpoints are not yet exposed in our REST API. However, there's plenty of support for this via our SOAP API. Currently, the plan is to look into exposing cashbooks and entries for Standard Journal (e.g. Danish agreements) in early 2016.
Meanwhile, look into the cashbook methods in SOAP:
https://api.e-conomic.com/secure/api1/EconomicWebService.asmx