ASM is hard to document as it is always changing.
Here is what I have:

• Estimating Economic Environmental Impacts At the National, Regional, and Watershed Levels: The Linked ASM/HUMUS Modeling System
• Scope of ASM: The Agricultural Sector Model ala 1992
• Dynamic Updating Procedures in the ASM ala 1996
• Basic Stochastic ASM
• ASM- Trade Modeli Function
• Big but old ASM manual
• Notes on Crop Rotation Modeling in ASM
• A Guide to Running Alternative Scenarios with ASM
• ASMGHG Document with Details on Greenhouse Gas Emission (Mitigation) Components ala 2000
• Brief Technical Summary of ASMGHG ala 2001