What do your pages look like?
I assume these are just HTML files and have a consistent template on all pages, no? For example, it has proper HTML markup along with HEAD and BODY, etc.
In this case, you can simply read it as if you were reading normal text files and doing some parsing on them to extract a part inside the BODY tags, and then you can just concatenate them and print them in ASP- # include style.
To get the actual file path name on the ASP.NET website, you can use Server.MapPath
var actualDiskFilename = Server.MapPath("~/somewhere/somepage.html");
System.IO ASP.NET, , :
var virtualDir = "~/somefolder/";
var actualDir = Server.MapPath(virtualDir);
var files = Directory.GetFiles(actualDir);
, ?