All EU law is derived from the Treaties, which form the primary legislation of the EU. Your studies may mean you need to consult the current, consolidated versions- to which the link below should be a quick link. (The historic, as originally promulgated versions are still available in the OJ)
The founding treaties were:
ECSC Treaty 1951 (expired 2002)
Euratom Treaty 1957
EEC Treaty 1957
Founding treaties expired or were changed and consolidated by amending Treaties, chief among which were:
Lisbon Treaty (OJ C 306 of 17 December 2007)
Treaty of Nice (OJ C 80 of 10 March 2001)
Treaty of Amsterdam (OJ C 340 of 10 November 1997)
Treaty on European Union, amended by the Maastricht Treaty (Consolidated version, 1992 OJ C 224 of 31 August 1992)
Single European Act (1986) (OJ L 169 of 29 June 1987)
(Note: because some of these treaties consist of a series of amendments, they can only really be understood when consolidated with the text of the original treaty)
Search terms to use in SOLO include
For specifically legal history
Law -- European Union countries -- History
Law -- European Economic Community countries -- History
For wider historical/political history:
European Union -- History
European communities -- History
European federation -- History
Pay attention to the locations of SOLO results, as you may well be directed to other parts of the Bodleian, those holding our collections of politics, international relations and/or history.
Constitutional Treaty and the Lisbon Treaty:
Signed in Brussels on 18 June 2004, the Constitutional Treaty required ratification by the Member States. Following its rejection by referenda in France and the Netherlands, EU leaders agreed in June 2007 to set up an Intergovernmental Conference to prepare a Reform Treaty. Its text was agreed at the Lisbon summit October 2007 (its common name being “the Treaty of Lisbon”) and required ratification by all member states. It came into force on 1 December 2009. It amends the current EC and EU treaties, but does not replace them.
The numbering of the current treaties is the result of two renumbering processes: Article 12 of the Amsterdam Treaty renumbered the articles of the Treaty on European Union and the Treaty establishing the European Community. Subsequently, Article 5 of the Lisbon Treaty renumbered the articles of the Consolidated Versions of the Treaty on European Union and the Treaty on the functioning of the EU.