The most important sources of information for larger than average libraries are the auction catalogues which were usually drawn up after the death of the owner. This source is not altogether unproblematic. Most auction catalogues do not contain the entire collection of the former owner and sometimes also include books of other provenance. The uniform system of classification often obscures the individual character of the collections. In only a few cases do we have library catalogues (a printed one in the case of Bolongaro Crevenna and a handwritten one in the case of Gerard Meerman). Additional information can be distilled from the writings of foreign visitors, such as those of the Swede Björnståhl in 1774/1775. Notarial estate inventories have not yet been studied for the category of large book owners.
Above all, private libraries remained a tool for people in intellectual professions and administrative positions, i.e. professors at universities or academies and grammar schools, and graduates of these institutions (the lawyers/regents, theologians/clergymen and medicinae doctores). A functional basis was a common factor in almost all libraries. The content of the libraries, however, usually had a wider scope, on the one hand due to the interests of the owner, on the other hand because the library also served as a status symbol. A large private library was not only at the disposal of its owner, but also enabled him to act as a Maecenas to others.
Besides books, drawings, prints, coins and medals, paintings, antiquities or object of natural interest were often collected as well. The library could also be a derivative of another collection, as was the case with bookseller and coin collectorPieter van Damme and art collector Ploos van Amstel.
About halfway through the eighteenth century, partly under the influence of French bibliophily, the systematic collection of old manuscripts and printed works as rariora came into being. A division was made between the collection of rare works (also called the cabinet) and the rest of the book collection. Besides the large universal libraries with their sometimes numerous incunabula, manuscripts and other rarities (Crevenna, Meerman, Romswinckel and to a lesser extent Fagel), smaller, more specialised collections were also established, a development partly facilitated by the sales by auction of foreign libraries in the Netherlands and the rise of the antiquarian book trade.
In these libraries there were few contemporary works; Latin and French were predominant over Dutch and the books had an international provenance.
The universal private humanities libraries of the second half of the eighteenth century seldom remained in existence for longer than a generation or two. Around the turn of the century they were no longer replaced by others. The rapid growth of the Royal Library and other institutional libraries at the beginning of the nineteenth century was in part due to the incorporation of parts of these private libraries.
