FYI this is one of 18 known surviving copies of Caxton's translation of the "Stories of Troy," not a completely unique item...except that every surviving print

FYI this is one of 18 known surviving copies of Caxton's translation of the "Stories of Troy," not a completely unique item...except that every surviving print or manuscript from this period really is uniquely valuable, taking into account marginal annotations, bindings, stop-press corrections, etc (which are often of greater academic interest than just another printed copy of a known text). It's crazy to think how many scholarly treasures might be sitting unknown in the private possession of rich collectors, things which could change academic views significantly were they available for study (for example, there are so few surviving music manuscripts produced in England in the later 15th century that any new discovery changes the lay of the land).