Though less familiar than "penny dreadfuls" or "shilling shockers", they have become the most rapidly appreciating collectable in the book market.
Why? Is it because, as cheap books sold to a newly-educated public, they made publishing history? Or because they are must-haves for collectors of all editions of authors such as Dickens, Trollope or Eliot? Or because their covers have artwork by famous illustrators such as Dickens's "Phiz", John Leech and Charles Keene?
Yes, to all of those; but their greatest value - a sign of the times, this - lies in their condition. That sounds like stating the obvious; after all, every collector knows that condition counts. But yellowbacks are collectable only in pristine condition; not just as books, but as ephemera that has miraculously survived - touchy-feely artefacts as fresh as the day they were bought to while away a journey on a puff-puff from Euston to Crewe. Displayed on shelves, their still-bright colours look as if they have just been delivered by time-travel.
Brian Lake, of the London bookdealers Jarndyce, who specialise in yellowbacks and will be offering them at next month's Chelsea Book Fair, says: "Yellowbacks that are merely `good for their age' are no good. They've got to be crisp and bright, with no rubbing. That's where they score, both visually and financially."
A scuffed yellowback can be worth as little as a tenth of the value of one in tip-top condition. He proved that in 1990, when he issued a catalogue of 258 pristine specimens and sold 95 per cent of them. These days, most prices are in the pounds 50-pounds 150 range - double his 1990 prices. The top prices, for a rare detective story or a rare original publication in yellowback, are between pounds 400 and pounds 500; again, double.
The bottom-line for a yellowback illustrated by Phiz is pounds 50-pounds 60 (in perfect condition, of course). Combined with a well-collected author, the price can soar above pounds 100. Interestingly, Mr Lake, who has a private collection of yellowbacks, would sell a Dickens for pounds 50-pounds 100 but would pay over pounds 100 for one that his collection lacks.
Who else is buying them? National collections such as the British Library - which is embarrassed by its old habit of re-binding acquisitions, now that collectors have recognised that the binding is part of a book's history. The British Library ruined stacks of yellowbacks by re-binding.
Next month, yellowbacks will be spotlighted when the bookseller WH Smith celebrates the 150th anniversary of its railway bookstalls. There will be promotions and a booklet, appropriately titled Time Traveller, which will explain the role of yellowbacks in publishing history.
Back in 1848, William Henry Smith II was awarded sole rights to the London and North Western Railway's bookstalls. He opened his company's first railway bookstall at Euston and soon had contracts sewn up with various other railway companies.
Yellowbacks were not sleaze. WH Smith II was known as "Old Morality" and he won his railway bookstall contracts because he promised to clean them up. Previously, they had been seedy cabins run by handicapped railway employees or employees' widows that sold unsavoury literature and dog- eared newspapers.
Yellowbacks were mostly popular fiction - romantic novels, detective stories - but they were also non-fiction such as guides to butterflies and moths, hints on etiquette, even advice on do-it-yourself taxidermy. WH Smith II was a force behind yellowback publishers such as Routledge - a pioneer in the yellowback market - Chapman and Hall, Blackwood and Bentley, buying up authors' copyrights and presenting them to the publishers for reprinting. He made available George Eliot's Felix Holt to railway passengers - and the fact that a novelist called Disraeli was published in yellowback probably had something to do with the fact that WH Smith II became secretary of state for the navy in Disraeli's government.
Crucially, yellowbacks at 2s were much cheaper than cloth-bound books that cost 7s or even 11s. Having been introduced in 1847 as The Parlour Library by the publishers, Simms and M'Intyre, originally based in Belfast, they were copied by Bentley's Standard Novels and by Routledge's Railway Library, and a publishing war developed. One result was that authors stopped writing three-volume novels that would be uneconomically bulky for railway bookstalls. Another was that illustrated covers came to stay - hence today's illustrated loose paper dust-jackets.
Today's yellowback connoisseurs cite a "golden period", 1855-1870, during which the cover artist - who was, oddly enough, usually commissioned by the printer, not the publisher - was allowed full scope to produce both picture and lettering, both on the cover and the spine. These yellowbacks have a unity of design that was lost when economics forced printers to standardise spine design and cover typography, reducing the artist's contribution to a dropped-in illustration. By the 1890s, yellowbacks were looking tawdry and decadent.
New collectors should be aware that the first editions of literary journalists such as George Augustus Sala, Edmund Yates, RB Brough, Augustus Mayhew and Douglas Jerrold are yellowbacks, as is much early detective fiction. Recollections of a Detective Police Officer by "Waters" (William Russell), published by J&C Brown in 1856, was a bestseller much imitated by other publishers. A "confessions" genre evolved from it that included, in 1860, The Confessions of a Thief, The Confessions of a Horse Dealer and The Revelations of a Catholic Priest.
But if you prefer to yawn and nod off during a train journey, I recommend the Fun Library's "Mrs Brown" series of topical absurdities, published in the 1860s. Mrs Brown is a bore.
Chelsea Book Fair, Chelsea Old Town Hall, Kings Road, west London, Friday 6 November (2pm-8pm), Saturday 7 November (11am-6pm). Entry pounds 2 or free tickets from Antiquarian Booksellers' Association (0171-439 3118). Jarndyce, Antiquarian Booksellers (Brian Lake), 46 Great Russell Street, London WC1 (0171-631 4220).
`Time Traveller' by Roger Williams, Cover Publishing, will be available from WH Smith from 6 November (pounds 2.50)