Paul Ricoeur noted that when we meet an author "we experience a profound disruption of the peculiar relation that we have with [them] through [their] work.” (Ricoeur 1991, p. 107)
Meeting an author (or an artist) removes the distance we normally have from them when we consider their books (art). A dialogue is possible where it wasn't before.
In the case of a book signing, however, it is likely to be an unbalanced dialogue which emerges. Loaded with the scripted thoughts of the reader/consumer and hurried by the queue behind, some of the distance remains.
I went to see the Gilbert and George postcard art exhibition at the White Cube a few months ago, and thought it would be interesting to see them in person at their book signing at the Tate last week - not least as they too collect old postcards. I went, after a day in the library.
Ricoeur, P., 1991. From Text to Action: Essays in Hermeneutics II. Evanston, Illinois: Northwestern University Press.