There are advantages and disadvantages to being signed up to the Advanced Book Exchange's electronic newsletter. The disadvantages include having lovely books you can never afford waved in front of you, as well as the results of irritating polls ('Funniest Books Ever') and updates to the latest bit of Harry Potter tom-foolery. The advantages are links to interesting bits of old book trivia, like this recent posting about the old 'Yellowback' books (sometimes called 'Mustard-plasters').
The yellowbacks were the airport fiction of the Victorian and Edwardian eras: cheap, mostly of insignificant literary merit, and with colourful, lurid covers. Routledges 'Railway Library' was the main purveyor of these thrillers, romances, travelogues and comedies, though some writers of quality did slip into their list (such as Robert Louis Stevenson).
As that ABE post notes, the yellowback style was created by designer Edmund Evans, who printed woodblock illustrations in brown (with two other colour tints) onto yellow glazed paper for maximum eye-catching potential at a reduced cost. (There's a small collection of his illustrations here.)
Anyway, this is as good an excuse as any to post some of my old yellowback cover scans (I do not, unfortunately, have many of the original books). Click for somewhat bigger versions (while pondering the tragedy that you don't get many titles like The Rajah of Monkey Island these days).
The middle book in that last row is Doctor Nikola, second of the five Nikola pulp novels by Adelaide-born Guy Boothby. Adelaide doesn't have many once world-renowned Victorian pulp writers to its credit, but fortunately Boothby was actually pretty good at what he did (which was to write classy tosh).