Code of Canon Law [1983]:
http://www.vatican.va/archive/cod-iuris-canonici/cic_index_en.html
New marriage law promulgated by Pope Francis on 8 September 2015:
http://w2.vatican.va/content/francesco/en/motu_proprio/documents/papa-francesco-motu-proprio_20150815_mitis-iudex-dominus-iesus.html
Instruction “Dignitas Connubii” [2005]: to be used by Tribunals in processing nullities: http://www.vatican.va/roman_curia/pontifical_councils/intrptxt/documents/rc_pc_intrptxt_doc_20050125_dignitas-connubii_en.html Catechism of the Catholic Church: http://www.vatican.va/archive/ENG0015/_INDEX.HTM
Supreme Tribunal of the Apostolic Signatura: http://www.vatican.va/roman_curia/tribunals/apost_signat/index.htm
Tribunal of the Roman Rota: http://www.vatican.va/roman_curia/tribunals/roman_rota/index.htm
Birmingham Metropolitan Tribunal [designated appeals tribunal for Scotland]: https://www.birminghamdiocese.org.uk/agencies/curial/marriage-tribunal/
Canon Law Made Easy http://canonlawmadeeasy.com/
A few straightforward books to help understand what marriage nullity is in the Catholic Church:
“Annulment: 100 Questions and Answers for Catholics”, by Peter Vere and Jacqi Rapp: http://www.amazon.co.uk/Annulment-100-Questions-Answers-Catholics/dp/0867168730/ref=sr_1_2?s=books&ie=UTF8&qid=1434704709&sr=1-2&keywords=annulment#reader_B005KMLOOO
“Annulments and the Catholic Church”, by Edward Peters: http://www.amazon.co.uk/Annulments-Catholic-Church-Edward-Peters-ebook/dp/B004EPZ0OI/ref=sr_1_3?s=books&ie=UTF8&qid=1434704709&sr=1-3&keywords=annulment
“Annulment: the wedding that was. How the Church can declare a marriage null”, by Michael Foster: http://www.amazon.co.uk/Annulment-Wedding-Church-Declare-Marriage/dp/0809138441/ref=sr_1_4?s=books&ie=UTF8&qid=1434704709&sr=1-4&keywords=annulment
“Marriage, Divorce and Nullity: A guide to the annulment process in the Catholic Church”, by Francis Morrisey and Geoffrey Robinson: http://www.amazon.co.uk/Marriage-Divorce-Nullity-Annulment-Catholic/dp/0814614299/ref=sr_1_7?s=books&ie=UTF8&qid=1434704709&sr=1-7&keywords=annulment