I have to do an essay on the characteristics and objectives of the State between the XIV and XV century and while I do have some base material I can work with, I was wondering if there is any other readings that you would recommend on this subject.
I would focus on the Kingdom of Aragon if you're interested in the Iberian peninsula. The archives for that kingdom are fairly complete and you can find lots of books with titles like Vanlandingham's Transforming the State: King, Court and Political Culture in the Realms of Aragon (1213-1387).
You can also find a bunch of useful primary resources that are published from the archives of the Kingdom of Aragon such as, Procesos contra los nobles de la Union aragonesa en 1301. Colección de documentos inéditos del Archivo General de la Corona de Aragón. If you can't read Latin or Spanish (the documents will be in Latin, the context will be in Spanish," I believe there is a massive four volume collection on James II published by an American scholar.
John Watts, The Making of Polities: Europe, 1300-1500, (Cambridge, 2009) should be your go-to starting point. The concept of the state is a slippery slope in the medieval period and I would suggest dedicating a paragraph or two to the definition and pertinent historiography (the debate between Susan Reynolds and R.R. Davies in particular)^1. Be wary of drawing shallow and teleological conclusions between the emergence of proto-state features and the inevitability of a state emerging. There was a shift in that direction in both England and France during the s.xiv only to be put on a knife edge in the s.xv. Another interesting case study might be Benjamin Arnold, Princes and Territory in Medieval Germany (Cambridge, 1991) although he's tough going in prose style. A final warning, don't be overly ambitious in choosing comparators. A good study will be able to balance historiography with discussion of evidence something that can be next-to-impossible with more than two or three comparisons.
^1 R.R. Davies, 'The Medieval State, the Tyranny of a Concept?', Journal of Historical Sociology, 16 (2003), pp.280-300; Susan Reynolds, 'There Were States in the Middle Ages: A Response to Rees Davies', ibid, pp.550-555.
/u/TheGreenReaper7 counsels well on the dangers of imposing modern notions of state backwards onto central-late middle ages. I would recommend the following as a helpful, if dense, guide to ideas/evidence of state in the middle ages. It's pretty learned if traditional legal/governmental history, mostly resisting problematic teleology:
Mostly France and England, some Germany.
To offer a contrast to the European-history replies, David Morgan's "Medieval Persia: 1040-1797" is a fascinating examination of what the "state" is and means for the major eastern empires on the periphery of Europe in the middle ages, and how it contrasts and compares with "feudalism" (which I put in quotes for the obvious toxicity of that term.)
Highly recommended, and it gets to your question immediately for the given area.
Any particular state or just states in general?