The French pronucuation and therefore the correct one is ''WELLBECK''.
But many TV and radio reporters worlwide outside France are mis-pronouncing his last name in a variety of ways, as listed above. You can HEAR these mis-pronunciations on several YouTube news site where Monsieur Houellebecq is interviewed or in news reports outside France.
on August 31, 2006
Houellebecq = WEL BEK
on September 5, 2006
''OO ELL BEK,'' but it'll sound like "WEL BEK" to most English speakers because the "OO ELL" is pronounced so quickly.
(The H is silent, and "ou" in French is like English "oo".)