General Information:
EU-Community: http://eu-community.daad.de/index.php?id=54#
EURES-Portal: https://ec.europa.eu/eures/main.jsp?acro=lw&lang=de&catId=490&parentId=0

Visa: 
Visa types and requirements: https://www.mfa.bg/en/services-travel/consular-services/travel-bulgaria/visa-bulgaria

Entry and residence requirements: 
Ministry of Foreign Affairs: mfa.bg/...

Working in Bulgaria:
Anglo INFO: bbs.angloinfo.com/...

Social insurance provisions:
National Social Security Institute: nssi.bg/...
Anglo INFO: bbs.angloinfo.com/...

Tax regulations:
Ministry of Finance: minfin.bg/...
National Revenue Agency: nap.bg/...
Bulgaria Income Taxes and Tax Laws: worldwide-tax.com/...
European Commission: ec.europa.eu/...

Accommodation:
Sofia Apartments: sofiaapartments.com/...
Bulgaria Rental Housing by Area: sub.let.com/...

Country information:
Wikipedia: wikipedia.org/...
Kwintessential - Doing Business in Bulgaria: kwintessential.co.uk/...
Culture Crossing: culturecrossing.net/...

Further information:
BBC country briefings: news.bbc.co.uk/...
Bulgaria Travel: bulgariatravel.org/...
Bulgarian tourist office: discover-bulgaria.com/...
Travel Information: travel-bulgaria.com/...