For me, I'm only interested in the novels, and it doesn't matter to me one whit if it's a pristine dragonskin-bound tome signed by Duke Samuel himself or a coverless dog eared paperback found in a gutter - the words are the same. Nearly all the rest is just junk - the maps, the cookbooks, the various editions, the models and commemorative egg-cups, the plays and movies and audio cds, - it all seems like bells and whistles to me and I fail to see the point of any of it. But I don't blame Transworld or the model makers or indeed Terry - business is business and if some suckers are willing to pay real money for this worthless crap then they're going to supply the demand and thank you very much, do come again.
By the way I do have a limited number of genuine London rocks I can let you have for a very reasonable price, all carefully numbered and ranging in size from a pebble to housebrick.