Plenty of literature coming out on the UK-EU relationship. Here I’ll quickly list four reports.
In January of this year the Czech EUROPEUM Institute for European Policy published a policy brief on what the EU can do to contain the risk of Brexit: http://www.europeum.org/en/eu-politics-and-institutions/107-analyses-articles-comments/2306-jan-vaska-what-can-the-eu-do-to-contain-the-risk-of-the-brexit
The LSE’s EUROPP blog is publishing a series I’m compiling made up of views from every other member states in the EU. The series began with views from Italy, Poland, Bulgaria and Malta. http://blogs.lse.ac.uk/europpblog/2015/09/30/european-views-on-the-uks-renegotiation-italy-poland-bulgaria-and-malta/
CEPS have produced an interim report on the UK-EU renegotiation: http://www.ceps.eu/publications/cameron%E2%80%99s-%E2%80%98renegotiations%E2%80%99-or-russian-roulette-eu-interim-assessment
A research briefing from the House of Commons Library on how the renegotiation is going: http://researchbriefings.parliament.uk/ResearchBriefing/Summary/CBP-7311