Very good links Mike. Thanks.
I am a bit obsessed with doors. I take photos of good old ones everywhere I go. It is well worth visiting old buildings (Igtham Mote near Sevenoaks has some great old doors) and places like the Weald and Downland Museum (which has many examples through the ages in many styles).
They are simple to make - as long as you can find really good oak or elm or chestnut nice wide and thick planks, it takes longer to hang one than to make it. It is well worth seeking and paying the extra for nicely forged, heavy duty ironmongery. Makes a big difference to the door.