El Bulli has been voted by chefs as the best restaurant in the world. The El Bulli cookbooks by Ferran Adria cover the back catalogue of their creations, which incidentally can be sampled at the El Bulli hotel, Benazuza. They are first published in Catalan and Castilian, versions in English become available later.

So to answer my original question, these books are worthwhile to a cook (chef?) who wishes to know more about the frontiers of cooking and are an eloquent souvenir of a trip to El Bulli restaurant or hotel. On the proverbial coffee table they would make a talking point amongst "foodies"  that would have few equals. 
 
 
 
 

ElBulli 2005 Ferran Adria  ElBulli 2003-2004  ElBulli 1998-2002  ElBulli 1994-1997  ElBulli 1983-1993             A day at El Bulli - Adria Over 1,000 photographs of everything that goes on at El Bulli with text inserts on a different sized paper, towards the end there are a fair number of briefly described recipes, you will see the menu, a plan of the restaurant, food order forms and even the outline of the booking policy that deals with the problem of the vastly oversubscribed bookings (2,000,000 for 8,000 places), everything in fact except how to get a table. But above all else this seems to be about photographs, pages of the El Bulli spoons, of the sea outside, of anything and everything.    Photos - El Bulli dishes

see also:-
Sous-Vide Cuisine - Joan Roja and Salvador Brugues
Another area of modern cooking tackled by fellow Catalan Joan Roja. This title tackles the low temperature vacuum packed method of cooking used in restaurants such as Roja's Moo and Celler de Can Roja as well as in "industrial" cook chill cooking. However these techniques require great care in preparation and expensive specialized equipment that I believe make this book only suited to the food professional. 

Adriá's foams
Adriá's foams revolutionised high end restaurant cooking in the mid 1990s. An essential part of copying them is the ISI Thermowhip, a whipped cream maker that has been developed to create cold and hot foams. The El Bulli cookbooks give instructions for quantities of gelatin, chilling/warming etc. but there is also an invaluable introduction to foams in The Cook's Book written by Adriá himself. The Thermowhip costs under £60 with a supply of canisters and is easy to find on sale on the web from suppliers such as Pennine Tea and Coffee or Carrick Leather goods (in UK).