When I was writing A Bird in Winter, my research consisted of two main activities: reading, and going on the run. Both were intensely pleasurable, of course, but as I went on the run with a rucksack on my bag containing everything I needed for a fortnight, there wasn't a great deal of room for books – it was mostly ordnance survey maps (my main character, Bird, goes on the run without a phone and I was trying to make my research trip as much like hers as possible).  

When I got home, and had had a good bath, it was a great pleasure to catch up with some of the books that helped me think about what Bird had done and who she was. She is working for the British Intelligence Services and has to go on the run when she realises that her boss is corrupt - but more than one set of people are on her trail, so as well as reading about spies I read about criminals and oligarchs - reading that made me realise there are whole layers of society of which we know very little. There's an argument for saying that all novelists are spies, but as well as my non-fiction reading, seeing how other writers had handled the complex layers of a spy novel was fascinating.