If you really enjoyed watching Secrets of the Castle, then you’ll be pleased to know that the DVD will be released on 26th January. You can pre-order the DVD here. Currently the DVD is region 2, so only playable in Europe, unless you have a region-free DVD player.
Also, a book about the building of the castle can be found on the Guédelon website.
It is currently out of stock (as I write), but you can put in your email address to be informed when it becomes available again.