I've used Git and CVS for that for my dissertation and for a couple of scholarly books I whose production I managed; great tools but not for the newbie.
What format were the source files in?
CVS and git are version control systems, but both implicitly expect the source document to be a
text file. You check in a document into the VCS. When you want to work on it, you check it out and do revisions. When you check it back in, the VCS does a diff on the new version, and saves what
changed as a delta to the original. You can reproduce the source document at any point by starting with the initial check in and applying deltas up the the desired revision point.
But as mentioned, the input is text files. The tools that do diffs and save only the changes from the previous version tend to fall down hard on binary files, which is what word processor documents have historically been.
MS Word (and Libre Office/Open Office Writer) now uses a form of XML as the underlying storage format, and you can theoretically do a diff on that, but it takes some doing. I had a go around with a professor in Britain using Word about it who said he couldn't find the XML file. No surprise, as Word wrapped it in a Zip archive, and he would have to find and open that.
There was a company called Component Software years back who claimed to be able to do version control using a VCS on Word documents and Excel spreadsheets. They no longer exist, and I haven't heard of anyone else trying to do it.
Can you talk a bit about what you did?
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Dennis