The basics about AB0 groups:
http://en.wikipedia.org/wiki/ABO_blood_group_system
http://anthro.palomar.edu/blood/ABO_system.htm
http://www.nlm.nih.gov/medlineplus/ency/article/003345.htm
A good video explanation:
english: http://www.youtube.com/watch?NR=1&feature=endscreen&v=oz4Ctau8mC8
spanish: http://www.youtube.com/watch?v=IrpQl9MnxjA
Testing AB0 at the Lab:
Tutorial: http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/intro.html
Video Procedure: http://www.youtube.com/watch?v=E0d5Qx42vmQ
http://www.youtube.com/watch?v=zj8ziUBMQhU
Book chapters:
English: http://jeeves.mmg.uci.edu/immunology/CoreNotes/Chap10.pdf
Spanish: http://www.edimeco.com/ws/component/docman/doc_download/13-7-8-banco-de-sangre-2
Associated patologies:
The antiglobulin test: