MDL Primer and Manual (1977) by Stu Galley and Greg Pfister. This documents the MDL interpreter version 54 (ITS) and version 104 (TENEX), and is an earlier version of "The MDL Programming Language" document that is also here.
The MDL Programming Environment (1980), by David Lebling.
The MDL Programming Language (1979), by Stu Galley and Greg Pfister. This documents the MDL interpreter version 55 (ITS) and version 105 (TENEX and TOPS-20).
The MDL Programming Language Primer (1980), by Michael Dornbrook and Marc Blank.