A starting point to create a web service?

Question

I have built an ERP for a small company using MS-Access (front end) and SQL Server 2008 R2 as database. Now one of their clients is implementing "SAP Business one" and I am asked to provide a web service for that SAP to enquire our database for stock availability.
I don't really know where to start. I have seen there are native web services in SQL Server, but MS seems to discontinue that.
From what I have googled, I understand that REST is not appropriate, because we want the service to be restricted to identified clients, so we would have to go for SOA and WCF ?
Is that correct or stupid ?
I am looking for links / books, or very simple code samples (if that exists).
I have already found Good starting point for learning to create ASP.NET SOAP web services and https://stackoverflow.com/q/296040/78522.
Any suggestion welcome, thanks.

---

Edit: just for the info: I have found these 2 links quite usefull, specially the 1st one, which is really "quick and practical", ideal for a total newbie in the field.
on w3schools.com
on siteduzero.com (in French)

Answer 1

I understand that REST is not appropriate, because we want the service to be restricted to identified clients.

This is not correct. REST services can have security, and most of them that I'm aware of do. The common ways to do this are:

• Use HTTP authentication (basic or digest). Most languages will already have libraries to handle this for you.

• Define some other way of logging in. Some REST services accept a username and password and return a cookie. Some use OAuth. Twitter is a good example of this.

• Don't use login at all, just validate that the client has some sort of token or password (probably sent as a cookie).

• Use any other form of security that works over HTTP.

Answer 2

Windows Communication Foundation

Windows Communication Foundation Walkthrough

https://stackoverflow.com/questions/386801/wcf-book-recommendations