It would be a good idea to look into the US National Security Strategy 2002 and analyse the impact in which it had on the definition of sovereignty; a good section of the essay should evaluate this piece of documentation.

A good structural idea in writing the essay would be roughly to keep to this:
Introduction – 312 words and Thesis – 50 words in the same paragraph

Road Map – 225 words discussing on what will be discussed within the essay

4 Paragraphs – 312 words each supporting the argument of the thesis

Anti thesis – 350 words discussing issues that go against the thesis, acknowledge and address possible criticisms of thesis

Conclusion – 312 words

Also you should be signposting between paragraphs by starting each argument by referring to the previous section.

Address the question or title; follow a structured and signposted sequence; demonstrate familiarity with the relevant literature to the concept of sovereignty; present an analysis and evaluation of the ideas and theories discussed; reveal internal integration and coherence; use references and examples to support the claims and arguments made; provide detailed references and sources in the bibliography or reference section; be written in good and grammatically correct English. Differences within the range are usually attributable to differences in the quality of analysis and evaluation and internal integration and coherence.

Good academic books to consider are:

Brown, C. (2006) Sovereignty, Rights and Justice, Polity

Chesterman, S. (2002) Just War or Just Peace?, Oxford University Press

Jackson, R. (2007) Sovereignty: Evolution of an Idea, Polity

Bain, W. (2003) Between Anarchy and Society, Oxford University Press

Booth, K. & Dunne, T. (2002) Worlds in Collision: Terror and the Future of Global Order, Palgrave

Cunliffe, P. et al (2007) Politics without Sovereignty, UCL Press

Chandler, D. (2010) International Statebuilding, Routledge

Chesterman, S. (2005) You the People, Oxford University Press

Hehir, A. (2013) Humanitarian Intervention: An Introduction, Palgrave

Hehir A. and Robinson, N. (2009) State-building: Theory and Practice, Routledge

Jackson, R. (1990) Quasi-states, Oxford University Press

Krasner, S. (1999) Sovereignty: Organized Hypocrisy?, Princeton University Press

Simpson, G. (2004) Great Powers and Outlaw States, Oxford University Press

Scheipers, S. (2009) Negotiating Sovereignty and Human Rights, Manchester University Press

Zaum, D. (2007) The Sovereignty Paradox, Oxford University Press