I'm unclear about what the status of history/classics dissertations are once they've been successfully defended. Once a person is awarded a PhD is their dissertation open for anyone to review if you contact the university, or does this vary from institution to institution? Are their situations in which a person can keep their dissertation private if they are planning on publishing a full book on the subject soon? Or is it not possible to keep it private because it is technically "published" as soon as you get your degree?
It may depend on the institution but generally when you file your dissertation you can put a hold on it for a certain period of time to prevent it from being made public. The hold varies from university to university I believe. At my institution it is up to 2 years. This is often recommended to avoid having your ideas for the first book "scooped".
This is not necessarily only to allow for publishing a book, per-se (since getting a book out in 2 years would be a bit of a push) but also so you can publish articles and the like.
If you don't put a hold on it, or after the hold ends, it is theoretically open to public viewing (though its not always easy to find unless you have access to college level research resources) most often online (via something like ProQuest) for more recent works, but also in micro-film or hardbound for older works.