Those who think the tubercular Robert Louis Stevenson was a weakling should perhaps reconsider.

News that Edinburgh University Press is to publish an unexpurgated edition of his remarkable journal, The Amateur Emigrant, sent me to hunt it down. I could only find the second volume of the first-ever uncensored edition, but even so I read with awe.

The Amateur Emigrant is a pithy and vivid record of the ten days Stevenson spent aboard the SS Devonia in 1879 and - as described in part two - an even more arduous train journey across America. Without his parents' knowledge (he said he had business to attend to that prevented him joining them on holiday in Cumberland), he sailed from Greenock to America to see what would become of his affair with his married lover, Fanny Van de Grift.

What he endured at sea, and then on the trek between New York and California, shows a backbone made of tungsten, even if much of the rest of him was in a perpetual state of collapse. On the ship - as on the train - the novelist opted for steerage instead of travelling with his own class. Partly this was dictated by the need to save money, but it was also to allow him to gather material for a travel book, Stevenson being "anxious to see the worst of emigrant life". Nor was he entirely parsimonious. He did pay an extra two guineas (basic steerage cost six) to make sure he got decent food and a table at which to write.

Even before he embarked he was feeling ill. As he wrote to his friend Sidney Colvin from his berth, the night before they sailed, "I seem to have died last night... I have just made my will... God bless you all and keep you, is the prayer of the husk which once contained R.L.S."

Plagued though he was by a sinister sounding "itch", perhaps dating from his dissolute student days, Stevenson was nevertheless well enough to observe the snobbery of his fellow passengers. Thanks to his unblinkered gaze, the resulting book was too shocking to be printed in full, and the first part was published, posthumously, with the offending passages excised. The novelist's remarks about the way the wealthy looked down on those in steerage, as if they were caged animals, were considered unacceptable, while his respect for the working classes in steerage was a slap in the face for the elite.

It was not until the late 1970s that the complete version of The Amateur Emigrant appeared, when a private edition of the original manuscripts, edited by Roger G Swearingen, was published in America. If the frankness of the second volume is anything to go by, the sea voyage will be fascinating. Of his fellow train travellers RLS wrote: "When I was ill coming through Wyoming, I was astonished, fresh from the eager humanity on board ship, to meet with little but laughter." Racism towards the Chinese (and others) sickened him: "They declared them hideous vermin, and affected a kind of choking in the throat when they beheld them."

This second part was published in magazine form in 1883, but it is astonishing to think it was a century before the entire text was made public. How times have changed. Today publishers would be delighted at such an expose by one of Stevenson's reputation. The story of the journal's suppression, however, shows all too clearly the sort of sanctimonious, hypocritical society Stevenson had long chafed against.

Edinburgh University Press's full edition of The Amateur Emigrant, edited by Dr Julia Reid of Leeds University, will be a most welcome addition to his oeuvre. It is to be published late next year as part of the New Edinburgh Edition of the Collected Works of Robert Louis Stevenson. In the meantime, to mark the author's birthday on November 13, Edinburgh World City of Literature will be giving away free chapters. This should whet the appetite and remind readers what a remarkable, stoic and free-thinking original RLS was. His body might have been frail but his mind was like a steel trap.