In 2005 he published Dr Johnson's Dictionary: The Extraordinary Story of the Book that Defined the World, a biography of Samuel Johnson's epochal A Dictionary of the English Language (1755). The first popular account of Dr Johnson's magnum opus, it "charts the struggle and ultimate triumph of one of the first attempts to 'fix' the language, which despite its imperfections proved to be one of the English language's most significant cultural monuments". Avoiding the more usual portrayal of Dr Johnson as "a lovable eccentric", Hitchings "keeps drawing attention to the unremitting intelligence that Johnson's lexicographical labours demanded, not least in separating out the ramifying senses of common words". Whilst declaring that "Hitchings's task is to rescue Johnson from Boswell's attentions" Will Self pointed out "The Johnson of the Dictionary was never known to Boswell, and as the older man was ill-disposed to animadvert on his younger self, Boswell got such basics as the great man's working methods on the Dictionary glaringly wrong. Not so Hitchings". The American edition was titled Defining the World: The Extraordinary Story of Dr Johnson's Dictionary and won the Modern Language Association's prize for the best work by an independent scholar in 2005.

In April 2008 Hitchings published The Secret Life of Words: How English Became English, a study of loanwords, calques and their cultural significance. Following the English language's history through "its debt to invasions, to threats from abroad, and to an island people's dealings with the world beyond its shores" the book examines its unbroken acquisitiveness..."but for all that [Hitchings'] true object is to reveal past frames of mind and to show how our present outlook is informed by the history squirreled away in the words we use". Instead of using history to explain language, Hitchings "picks words apart to find their origins" and then molds this "mountain of dense information into an elegant narrative". The Economist noted that "whatever is hybrid, fluid and unpoliced about English delights him". The book was published in America under the same title the following September.

Hitchings' third book, How to Really Talk About Books You Haven't Read, a guide to books and literary erudition, was released in October 2008.

His fourth, The Language Wars, will be published in February 2011.

In November 2008 The Secret Life of Words won the John Llewellyn Rhys Prize, the first work of non-fiction to do so for six years. The chair of the judges, Henry Sutton, described it as "a landmark, 'vast' in scope and 'written with an unnerving precision, clarity and grace'," adding "amazingly accessible, it's written with great grace and enthusiasm and humour, and is also a scholarly work." The shortlist had also included the winner of the 2008 Booker Prize, The White Tiger by Aravind Adiga.

In March 2009, on the strength of The Secret Life of Words, Hitchings was shortlisted for the Sunday Times Young Writer of the Year Award. In June 2009 he received a Somerset Maugham Award.