One of my sister's acquaintances wrote 2016 mains with PSIR optional. She had been working in an IT firm till a month before prelims and had been preparing for a year before. 

Political Science and International Relations (PSIR) is quite fast. The study material provided by IGNOU is good for selective reading for specific topics like Justice, Equality, MN Roy, Sri Aurobindo, Theory of international relations etc. But she had an engineering background and revealed that the notes were not of premium quality.

Paper 1- Part A

Gauba is not bad. But you will need to read the book more than one time to interpret it completely. The information has been divided unevenly within the chapters making it a difficult read. One more good source is Shubra Ranjan's notes. They summarise the points in the UPSC syllabus format.

Paper 1- Part B

Complete Laxmikant.Read the interpretations given by famous analysts, their viewpoints, case laws and their history and the politics that played behind the evolution of laws and how and why the judiciary evolved. Again for this look into Shubra Ranjan notes. For the remaining topics look into Oxford publications.

Paper 2- part A

For this part, the best recommendation is Heywood’s global politics. It’s easy to understand and very well written. Combine this with IGNOU notes and you will get good results. You can also go through questions given in Baylis and Smith because sometimes they appear verbatim and quoting the exact same viewpoint can get you some extra points.

Paper 2 - Part B

Conduct internet research and make your own notes on the topics included in the syllabus. Then go through commentaries on IDSA /IPCS websites. This will give you a better understanding of and current status of relations.

Once you are done with all the study materials, focus on developing the vocabulary for PSIR. Make sure your answers are evenly distributed with theories, quotes, facts, years etc. Practice writing answers a lot.