The Other Glee Girls Whod You Rather - Trending Celebrities
The string an other is vanishingly rare in english. In contrast another is positively pervasive. I think it would be fair to say that the second has eclipsed the first to the point of making the first.
There's a formula: Another = an + other. Think of it as of an article plus the word other that have historically merged into one word. Grammar requires some article before other book;
