And why? Value may be construed in whatever terms you like (sentimental, monetary, scholarly, etc.).
I would have to declare a three-way tie for me:
1) A personally (and personalized) autographed copy of Walter Brueggemann’s Theology of the Old Testament: Testimony, Dispute, Advocacy (I have two additional personal and personalized autographed volumes by him as well).
2) A personally (and personalized) autographed copy of E. P. Sanders’ seminal Paul and Palestinian Judaism (I also have autographed copies of his two volumes on the historical Jesus; I had him sign these for me while I was at Duke; he retired my second year there).
3) An autographed copy of the novel The Oath by Holocaust survivor and Nobel Prize winner Elie Wiesel. This was a gift from a friend about 10 years ago, though I trust it is authentic.
Hopefully by the end of the year I will have a fourth book to add to this list, when my own, first book, is officially released with Eisenbrauns!