This ad is unique for several reasons:
First, it makes no mention of the type of book it refers to (fiction? non-fiction?), let alone what the book might be about or what readership it targets. There’s also no mention of the book’s publisher, which is highly unusual, given that in most early ads for Montgomery’s early books, the “From Page’s List” logo appears prominently almost always.
Second, the first appearance of this ad that I’ve found is in the 9 July 1908 issue, which is less than a month after the book was published (on 13 June). That means that the designation “cleverest book of the year” seems a bit premature.
Third, this ad appeared again and again in the months to follow: on 20 August 1908, 17 September 1908, 8 October 1908, and 15 October 1908, and then again on 15 July 1909, 5 August 1909, 2 September 1909, 16 September 1909, and 7 October 1909. Not only does it seem odd for this ad to be deemed the “cleverest book of the year” in two separate years, but the later ads show no awareness of the publication of Anne of Avonlea, early in September 1909.
In short, while this ad campaign breaks from a number of advertising conventions, clearly it was deemed to be effective, or else it wouldn’t have run for a minimum of nine times over a fifteen-month period. These ads may not have given anything away in terms of the book’s contents or form, but by using the term “clever,” thus commenting on the book’s literary quality, this ad campaign would have appealed to readers searching for “clever” fiction who may have turned away from ads proclaiming the book’s staggering popularity.