Manuscript to Type
Before printing with moveable type was introduced in the fifteenth century (in Europe) books were produced in manuscript form; every word had to be handwritten by accomplished scribes, often monks. This was a laborious and expensive process; copies of books were few and the cost of buying a book was prohibitive to most people. Books were owned by aristocrats and other rich people of the middle ages. They were luxury goods.
Though the Chinese had invented a system of typesetting as early as the twelfth century, it was not until Gutenberg set up his presses in Cologne in the mid fifteenth century that the technology came to Europe.
Mass Market
New technology spelled profit and opportunities for business. Suddenly the cost of producing a book was relatively low. The market was widened to include the burgeoning middle class. There was gold to be had and William Caxton, a successful English businessman, 
saw a way to broaden his empire in trade. Caxton (c1415 to c1492), after his apprenticeship to a mercer, had sought his fortune in Bruges and Germany. By his middle age he had become well off and highly respected as a merchant. He saw what was happening at the Gutenberg press and set up his own operation in Bruges. This was his testing ground before he moved back to London to establish his printing works in Westminster.
Caxton’s Press
It is a little misleading to think of Caxton as a printer in the inky-fingered sense. He was a printer in that he established, owned and ran a highly important printing business. But woodcuts and engravings of him may give the impression that he was a technician
, a man who worked the press; maybe he could, but his work as a translator, publisher and entrepreneur were what made his contribution to the development of printing so important. He was the first printer in England of books in the English language and his impact on the growth of the importance of English as an international language was immense.
Proofreading Accuracy
In Caxton’s time the English language was not standardised; there were many acceptable variant spellings. Though the new technology began to make inroads in this direction, it caused almost as much confusion as it relieved. (Standardise/standardize: the problem doesn’t quite go away!)
For speed, Caxton’s team of compositors (typesetters) and proofreaders worked together on the same books. Books were divided into chunks (signatures) which were worked on simultaneously by many people. Each had his own ideas about spelling and language. They had no dictionaries. Proofreading would be done in pairs; one man would read the text aloud (the reader), the other would correct the text (the corrector). The accuracy of this process (which was used in some printing houses until thirty or forty years ago) was not absolute! A man’s reading speed might have been too great for the corrector; unfamiliar words, foreign words and those with variant spellings all caused inaccuracy. Printers would learn to proofread on the job with varying degrees of success (like now).
A common peculiarity was the addition of a final e to a word. This might be done to even up (justify a line of text). But for many years afterwards many words were alternatively spelled with or without the final e (e.g. Englissh/Englisshe, good/goode).
The situation was not helped by the fact that many proofreaders were Flemish. Caxton translated many texts and brought some of his best men to England with him.
Then, as now, speed was of the essence in publishing; errors slipped through but they probably didn’t affect the sales of the new and wonderfully available printed books. A market which paid no attention to variations in spelling would hardly mind a few typos. Now, books that are strewn with errors get short shrift from reviewers and readers; good proofreaders are vital to the publishing industry and in demand both for hard-copy books and e-books.
Caxton published over a hundred titles, an immeasurably important achievement. His most successful books were probably Malory’s Le Morte Darthur and Chaucer’s Canterbury Tales. He published two versions of the latter, but may have made more money out of the second when he employed cost-saving methods like reducing paper size, and added illustrations. Nothing changes!
(One for the sharp-eyed: why did I add a comma after size in the line above?)
And: Le Morte D’Arthur or Le Morte Darthur? We’ve gone for the original here though the apostrophe version is more common now and the problem with the original is that most people think it’s a typo. Be honest, did you notice?