There are lots of compelling answers to the question, "Why book history?" Most of them would quote Don McKenzie, I'm sure. His stuff about how the medium effects [with an 'e'] the message and about how we have to concern ourselves with the relation between form, function, and symbolic meaning (Bibliography and the Soc of Texts, page 10). Maybe his claims about how documents bear the evidence of their own making (Making Meaning, amplified by McGann's recent New Republic of Letters). McKenzie is now the patron saint of bibliography.
Today, the New York Times's David Brooks offered a more urgent but presumably unconscious answer to "Why book history?"--unconscious in the sense that he's thinking about media and culture but not books in particular. Writing about an event that, given the pace of things, may be swallowed up within a few days' time, Brooks observed that "the crucial thing is that the nation’s culture is now enmeshed in a new technology that we don’t yet know how to control."
This is exactly why book history, for at least two reasons. First, Brooks's claim about the effects of social media on a culture (however accurate) suggest the ongoing relevance of book history. Some grasp on how people in early modern England reacted to and exploited the proliferation of printed documents is not merely instructive but necessary to comprehending the shift Brooks describes. Plenty of folks have written about this, of course, but it's worth saying again. Second, Brooks's claim--expressed with a combination of capaciousness (THE nation's culture!), urgency (NOW! NEW!), and determinism (YET!)--falls into a long tradition of people trying to make sense of the media transformations of their time. I'm reminded of the 1637 proclamation, A Decree of Starre-Chamber, Concerning Printing (STC 7757), which shows the Star Chamber, in its attempts to regulate the press, try to make sense of the social effects of a media transformation. Didn't work out so well for the Star Chamber.