If you need to read from a file (ex txt, html) located in SharePoint Document library, you need to get the SPFile object and then call the OpenBinaryStream from that file. here you can find a simple example that shows how to read from a html/txt file uploaded in a SharePoint Document library (it works too with SPLists):
string content = string.Empty;
using (SPSite oSite = new SPSite("http://localhost/"))
{
using (SPWeb oWeb = oSite.OpenWeb())
{
SPDocumentLibrary doclib = (SPDocumentLibrary)oWeb.GetList(DocLibUrl);
SPFile htmlFile = doclib.Items[0].File;
using (System.IO.StreamReader reader = new System.IO.StreamReader(htmlFile.OpenBinaryStream()))
{
content = reader.ReadToEnd();
}
}
}