Well I must say that seeing so much opinionated responses on this subject leaves me a bit saddened.
First off I'll post this link for those who want to read it. http://astelier-medieval-du-cuir.over-blog.com/article-21921258.html Might I suggest using Google chrome, it has a built in translator. The website overall has some great studies. Many of them I have tried, though I use rabbit hide glue, which comes in granular form and you must prepare it for it to be used. You can get it at http://www.naturalpigments.com/search_results.asp?txtsearchParamVen=12 Just scroll down, they have a variety of glues. There are other places that sell it as well online, just look for animal hide glue, there is also fish glue.
Cuir Bouilli as ChuckBurrows has mentioned is still very much used today. And there is some loss of information that we are still trying to recover. See, most medieval and earlier leather working was done under what are known as guilds. And most of that information was passed down verbally. Only around the Victorian times did leather working information become available in book form. There are a bunch of good free books online under Google Books.
Rabbit hide glue has been used with leather for a long time. It is still used today by master wood workers that do restoration on wooden items and some leather items as well. And some of us crazy people who choose to study it's history. And as for Cuir Bouilli and it's definition.... Keep an open mind. One place may have thought hot water was used, and another place hundreds of miles away may have used cold water (leather bubbles either way). And the application of sealers/hardeners could range from wax to glue to varnishes (shellac is ancient along with hide glue).
Remember to have fun and keep an open mind. I don't think there was a one and only way to do this. Different applications for different results.
Enjoy, play and learn. It's what makes us the great teachers for the next generations.