In 1797, Thomas Wilson wrote in "The Use of Circulating Libraries": "Consider, that for a successful circulating library, the collection must contain 70% fiction". However, the overall percentage of novels mainly depended on the proprietor of the circulating library. While some circulating libraries were almost completely novels, others had less than 10% of their overall collection in the form of novels. The national average start of the 20th century hovered around novels comprising about 20% of the total collection. Novels varied from other types of books in many ways. They were read primarily for enjoyment instead of for study. They did not provide academic knowledge or spiritual guidance; thus they were read quickly and far fewer times than other books. These were the perfect books for commercial subscription libraries to lend. Since books were read for pure enjoyment rather than for scholarly work, books needed to become both cheaper and smaller. Small duodecimo editions of books were preferred to the large folio editions. Folio editions were read at a desk, while the small duodecimo editions could be easily read like the paperbacks of today. The French journalist, Louis-Sébastien Mercier wrote that the books were also separated into parts so that readers could rent a section of the book for some hours instead of a full day. This allowed more readers could have access to the same work at the same time, making it more profitable for the circulating libraries.