There's an excellent article by Wayne Gooderman up at the Guardian Books Blog at the moment, titled "The secret stories of book inscriptions".
I've always preferred secondhand books to new ones. The pages have a lived-in feel, while hints of previous existences often appear in them, such as train tickets or postcards, once used as bookmarks, enabling the imagination to speculate.
However, as Gooderman's article points out, the most intriguing aspect of secondhand books is often the bespoke dedications that we encounter in them, leading us towards stories beyond those told by the texts that follow. I personally find the dedications from grandparents to grandchildren the most poignant ones when I'm searching in charity shops for books for my son. These discards of adolescence are charged with the concentrated expression of love by elderly people. I invariably buy them because books, like people, deserve a second shot at love.