Migrate internal B2B WebService to web API?

We've developed a webservice (WS) to enable our desktop management app to communicate with common market ERPs, the WS is composed by several projects each responsible to extract data from and to the ERP and our application. With our growth, more and more we have the need to supply our partners with an API so they can develop customized integration. My idea is to migrate to web api due to its expansion, but i'm having trouble to map the complexity of business logic to , for example REST Architecture, with the GET, PUT , DELETE verbs. Can anyone point me in the right direction?

Other thing i was thinking was developing two Api's:

 

a) one for extracting and inserting data from and to our app (using JSON or XML)

b) other to transform JSON or XML to a specific market ERP, using an improved version of the above mentioned projects.

 

Again here, i face problems like business logic processing, like inserting a sales doc that will link with an order in the external system, or flexible data related with a client customized versions of that ERP.

 

I don't expect that you give any straight answer, other than share with me some insights or interesting articles, blogs, documentation where i can start to digg and build some of our documentation and approach.

 

Thanks