In case you can’t quite make out the text: this graphs shows the relative frequency of glamour (blue line) and glamor (red line) in American books published between 1988 and 2008.
In case you can’t quite make out the text: this graphs shows the relative frequency of glamour (blue line) and glamor (red line) in American books published between 1988 and 2008.
notoneoffbritishisms.com 
Maybe you should “blame” Glamour magazine?
If you look at a longer-term Ngram, you can see that the our spelling in the U.S. steadily increased from the early 1800 and reached an all-time high in the 1930s, which was when Glamour Magazine was founded (1939)! Then it tailed off till about 1990 (which I claim as the start of the current not one-off Britishisms trend) and has been rising steadily ever since.
Whenever I see the American spelling “glamor” I tend to want to associate it with “clamor”–some sort of ruckus–whereas I think of fashionable luxury with “glamour” and “glamourous.”
(BTW, during my formative years in the US, I read almost exclusively British authors from 1989 on, but, as a spelling/copyediting nerd, I paid attention to the dialects’ differences. Between my reading and regular, extensive consumption of British broadcasting, I wound up forming an idiolect that has gotten (Amer.)/has got (Brit.) me accused of being Canadian or English by less-tutored American ears.)
But nobody would write “glamourous”, surely?
The word “glamour” was introduced into the language by Sir Walter Scott. It is a Scottish variation of “grammar” and has nothing to do with the -our endings of words that Americans drop the “u” from. “Glamor” is therefore just plain wrong. The connection between “grammar” and “glamour” is that the first was originally the magical spell cast by a combination of words; the second is the magical spell cast by a combination of personal qualities.