An association copy is a book that has enhanced importance because of the person or people who owned it. It may be presented by the author or artist to someone who is important to that person or the work itself.

In some interesting cases, an association copy may include writing by the author or artist with annotations or marginal notes of relevance to the work.

An example of the latter would be the author’s copy of a work that was marked up for changes to be made in a future edition.

ABC for Book Collectors by John Carter provides an interesting definition with some examples:

Association Copy
Association Copy

Other People’s Books by G. Thomas Tanselle for the Caxton Club of Chicago is a hardcover illustrated exhibition catalog that has an impressive selection of association copies which tell important stories.

Book Cover-Other People's Books
The Caxton Club of Chicago put on an exhibit of significant association copies owned by their members. This is the catalog for that exhibition.

An example of an Edward Stratemeyer association copy is the book he signed and sent to fellow author, George Waldo Browne. Earlier Browne was instrumental in recommending Stratemeyer as a successor to “Oliver Optic” which led to Edward writing An Undivided Union (Lee & Shepard, 1899). Browne was also a frequent correspondent with Stratemeyer as they exchanged information on the state of publishing and other details of mutual interest. Browne wrote a couple stories that Stratemeyer owned, including Jack North’s Treasure Hunt (Chatterton-Peck, 1907).

Under Dewey at Manila by Stratemeyer
Edward Stratemeyer signed and presented this copy of Under Dewey at Manila (Lee & Shepard, 1898) to a fellow author, George Waldo Browne in exchange for a signed copy of one of Browne’s books.

As noted by Carter, the label association copy can get overused. A book signed by Stratemeyer might be scarce since he did not participate in author signing events but a mere signature is not an indication of an association copy.  

Even a work signed and presented to a family member would usually not qualify. However, when the recipient had some significance to his career, then the copy has more interest as an association copy.