We have recommended http://www.knackered.org.uk before for a very clear explanation by David Green (former ERS Council member) of the disadvantages of First Past The Post and advantages of the Single Transferable Vote.
We also recommend The Politics of Electoral Reform by Michael Meadowcroft. He is a current member of the ERS Council and a former Chair of it. His book does a similar job to David’s publication but in more detail. You can visit http://www.bramley.demon.co.uk/currentaffairs/politics_of_electoral_reform_2015.pdf to see the 4th edition of it.