UKVI’s European casework instructions are here https://www.gov.uk/government/collections/eea-swiss-nationals-and-ec-association-agreements-modernised-guidance
The government’s statement of intent about the settlement rights of EU nationals after Brexit is here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/718237/EU_Settlement_Scheme_SOI_June_2018.pdf
The website of the Court of Justice of the EU is here: https://europa.eu/european-union/about-eu/institutions-bodies/court-justice_en
EurLex is a site giving access to EU law of all sources: https://eur-lex.europa.eu/homepage.html EurLex is, as its home page says, the portal giving free access to European Union law. It is searchable for treaties, case law, legislation, and legislation in preparation. The latter can be tracked through the process of the European institutions and you can see how new provisions adopted under the Treaty of Amsterdam are amended as they progress.
http://europa.eu is the general site for the European Union, and a source of policy statements from the European Union bodies on issues such as enlargement and asylum policy.
The European Council on Refugees and Exiles www.ecre.org is an umbrella organization 'working towards fair and humane policies for the treatment of asylum seekers and refugees'. It is an extremely useful source of informed and up to date opinions on European refugee law and policy, as well as links to external material.
Statewatch www.statewatch.org is a Europe-wide body monitoring civil liberties and human rights issues, with very good coverage of the effects of these on immigration and asylum.