In 2016, Harding published "A Very Expensive Poison", an account of the murder of the Russian ex-KGB whistle-blower and Putin critic Alexander Litvinenko. The book garnered a positive response from reviewers, including from "The Spectator", "Guardian", "Times", and "London Review of Books"; Robert Fox, writing for the "Evening Standard" described it as "one of the best political thrillers [he had] come across in years". Lucy Prebble adapted the book for the stage. A production ran at The Old Vic theatre, London, from August to October 2019.