Oxford Professor Michael Broers has already published two books of his trilogy on Napoleon Bonaparte. He wrote that he chose to write this biography because new sources had become available. On the first page of Philip Mansel's new biography of Louis XIV, he writes that he used a recently published correspondence with Louis's wife Madame de Maintenon. How is it the case that these sources, which are hundreds of years old, are only becoming available today?
Step 1: Stuff Goes Missing
If there's a source historians don't know about, it's because it has been lost at some point. People stash things and forget where. Libraries move to a new location and a box of documents gets left in a corner by accident. A medieval archive took bad records at the time and now nobody fully knows what's on some of the shelves (Vatican Library, I'm looking at you here). It's very easy for documents, even really important ones, to go missing. It's also possible for documents that were meant to be destroyed to survive, such as if they were saved and stashed by a conscientious staffer or went missing before they could be destroyed. It's very easy for things to get lost. And sometimes, things are hidden on purpose.
Step 2: Stuff Gets Found
So then, hundreds of years later, a historian is trawling through a local archive and asks the question "I haven't found anything yet, do you have any uncategorised boxes?", or an archivist is sorting through the inventory and goes "Huh, I don't remember this box being here" or "Hang on, this letter is under the wrong date, we've got a couple of folders mixed up". In my own area, the Middle Ages, the vast majority of lost records were rediscovered in the 19th and early 20th century as part of a concerted effort to trawl through government collections and local archives to find as much as possible. They went around every monastery they could find, and thousands of documents were discovered, including long lost poems and narrative accounts of crusades. It was all there sitting on shelves or in boxes, but wasn't found until someone thought to ask about them. More recently, things get found by chance that weren't logged even in previous sweeps for documents. For example, in the 1970s, a cache of around 130 love letters from the 12th century were discovered. More recently, a copy of the Jubilee Book, a document containing 14th century London's laws and constitution that was deliberately destroyed, was discovered copied into an old manuscript sitting in Oxford. In that case, it seems like the document was deliberately hidden, because the book was supposed to be destroyed. Someone wanting to preserve it copied it into a totally unrelated manuscript so it wouldn't be found until long after the political crisis cause by the book was over.
Step 3: Stuff Gets Transcribed And Published
But finding it doesn't mean historians can use it. For that to happen it has to be transcribed and then published as a more accessible modern edition. For example, the Jubilee Book was found in the late 20th century by Caroline Barron. It's a really important document; the product of a two year inquiry into the governance of the city of London conducted in the 1380s that produced a series of legal and constitutional recommendations. That a copy survived has been known about for 20 years, and for that 20 years Caroline Barron has been doing the following:
This is a long process. Barron's edition of the Jubilee Book comes out this year. It can sometimes be skipped, especially if the new source is a short document that can just be copied into a much larger book.
Step 4: Historians Use The Stuff
But once a source has been rediscovered and republished in a modern edition, we can use it! And that's how it comes to be that historians write books using "newly published" sources. The sources may have existed for hundreds of years, but that doesn't mean we had access to them. Once they have been published in a modern edition, nicely translated into English with accompanying notes, we can use them. And sometimes, that means we have to write new editions of existing work or just write a whole new book.