Books traded in the Netherlands were not identical to books produced in the Netherlands. A substantial percentage of books distributed here originated from abroad and books produced here were partly intended for export while some publications remained outside the book trade, such as most government publications and other predecessors of the 'grey literature'. Nor is the book sold in the Netherlands identical to the book read in the Netherlands, although this distinction is harder to maintain. The sources for both categories do not abound and, if available at all, are rather one-sided and difficult to interpret. Moreover, they are often identical or blend into one another without clear distinction. An example is the phenomenon of the 'auction catalogue of a private library' (see 2.4.6): at first glance such a catalogue seems to be a fine source of information on what an individual read but on closer inspection it reveals, in the most favourable case, a summary of what the owner possessed and was even then often incomplete or, on the contrary, complicated by additions. In fact, the only thing we can be certain of is that it was a collection of books for sale at one particular time and place.

During the period under consideration, auction catalogues of private libraries form the richest, or at least the most numerous, source of which books were in circulation in the Netherlands. Currently, about 1800 catalogues are recorded in the database of the Nederlandse boekhandelscatalogi tot 1800 project. Publisher's lists and wholesale and retail stock lists add another 500 numbers. Catalogues and advertisements used by publishers to fill a book's blank pages came into fashion in the Netherlands in the second half of the seventeenth century: in 2008, the STCN had gathered and copied a collection of about 1000 for the period 1668-1725. No booksellers' archives are extant for this period; newspaper advertisements also start to become more numerous from about 1670, but are not easily accessible and have hardly been studied systematically.

A survey of books sold in the Netherlands between 1585 and 1725, classified according to language, genre, form or format, can, therefore, not yet be given. The identification, tracing, and collecting of source material has started, as well as providing access to it, but in-depth studies or general overviews are almost totally wanting. Whether the accepted fact that it is much more difficult to distinguish between the retail trade, the publisher and the printer in this period than later will make research more difficult or not cannot yet be predicted.

According to a survey of the STCN, 380 books, not counting pamphlets and other topical and occasional writings, were published in the Netherlands in 1650. Of those 380 books, 54% had Dutch and 36% Latin as their main or only language, 4% were in folio, 25% in quarto, 30% in octavo, 32% in duodecimo, and 9% in other, mainly smaller formats. Further research will have to decide whether these figures say anything about books sold in 1650, let alone in 1600 or 1700. This exercise does, however, give the future researcher one useful lesson in statistics: if pamphlets and government publications (500 in all) had been counted indiscriminately, the total number from 1650 would have more than doubled to 880, and the percentages for Dutch language and quarto format would have risen from 54 and 25 to 71 and 63 percent respectively while the others would have declined proportionally. These additional 500 'books', however, would have constituted no more than the amount of printed paper necessary for one large Bible in folio format.