Matthew Flatt, "Composable and Compilable Macros, You Want it When?"
But, read all the PLT papers, and the old Scheme and Lisp ones! Also the paper @samth recently mentioned:
"Advanced Macrology and the Implementation of Typed Scheme", by Ryan Culpepper, Sam Tobin-Hochstadt, and Matthew Flatt has section "3.2 Modules, or You Want it When, Again?"