Suggestions & CommentsMARC21 Format Documentation
MARC21 Standards - Library of Congress
Country Codes
2-3 letter codes used in fixed field Ctry
Language Codes
3 letter codes used in fixed field Lang and variable field 041
MARC Basics 
- Linda Seguin
A very basic introduction to MARC bibliographic format
Understanding MARC Bibliographic - Library of Congress
MARC21 Tutorial - University of Southern Mississippi