Ok, first I am going to assume that you have the code for the connection "con" somewhere else and this is just a snippet of what you have.
Secondly, why the 'var' and semicolon syntax? Are you writing this in JScript?
Your problem is that you are including a, b, c, d, e, f into a string literal. As such, they are letters, not variables. You need to concatenate them into the string.
Try this...(in VBScript)
<%
Dim a, b, c, d, e, f
Dim sSQLString
a=Request.Form("LastName")
b=Request.Form("FirstName")
c=Request.Form("Street")
d=Request.Form("City")
e=Request.Form("Country")
f=Request.Form("Phone")
Response.write (a & " " & b & " " & c & " " & d & " " & e & " " & f)
sSQLString = "INSERT INTO tAuthors (LastName, FirstName, Street, City, Country, Phone) VALUES (" & a & ", " & b & ", " & c & ", " & d & ", " & e & ", " & f & ")"
con.Execute(sSQLString)
%>