One of the issues relating to a hung parliament is the question of whether the electoral system should be reformed.
Here are some starting points for considering the impact of different voting systems on election outcomes.
The main organisation calling for change in the UK is the Electoral Reform Society. It includes a large collection of online pamphlets and reports However, do remember that the ERS favours the adoption of single transferable voting.
More balanced Summaries of the the Independent Commission on the Voting System
and other Uk government reviews can be found in House of Commons Library publications. see this example from 1998 and search the site for others.
For a broader overview of the types of electoral system available, an excellent resource is the ACE electoral knowledge networkwhich has involvement from a number of leading organisations. It includes an encyclopedia of terminology, electoral laws and resources.
Another excellent resource is International Idea's Handbook of electoral system design Which examines different systems and their consequences.
In 2006, The Minority Rights Group also published a pamphlet on Electoral systems and the protection and participation of minorities which you might find useful.
Finally a resource (in favour of PR ) where you can find articles and papers supporting Proportional representation is the US based PR Library