Right now it’s typical for a history department to require the publication of a book for tenure—some places, like my own institution, will accept five peer-reviewed articles (which basically means you can cannibalize your dissertation). Writing a serious book in six years (the average time for tenure review) is no mean feat, but keep in mind that every newly minted Ph.D. has already done most of the research for his or her book when the tenure clock starts. It’s just a matter of revising the dissertation into a book. Not easy, but then again, not a project that necessarily demands six years. It’s perhaps for this reason that some universities are starting to demand the publication of a book and “significant progress” toward a second.