That's an strange concept, I think to myself. (page 66 of paperback edition)It is possible that the author initially typed an instead of a, and no editor caught it.
But maybe an was correct in the sentence's original form. Perhaps instead of strange, the author first described the concept as odd or unusual, in which case an would be appropriate. Then in a revision the author or editor changed the adjective to strange but neglected to change the article to a. That seems more likely to me.
Needless to say, Evelyn Wood's acolytes would not pause to ponder such editorial mysteries.