A really good source of reference books where you may find something on the item(s) you collect. FYI: the really old collector books may not have the manufacturer name or specifics of a newer book; but if you find your item in an older book, you KNOW it is not a "knock-off", something recently copied or reproduced. These are the books most quoted in accurate appraisals. They are rare because of it and be prepared to pay a higher price.
Other books you may find are on specific subjects, like music or art, or classical fiction books for adults and children that are beautifully illustrated with engravings (before 1880) or drawings by a known illustrator in lithography prints.