An interesting look at the cover art used for the Narnia series throughout the years over at Bookslut (please note: site name and logo are a tad risqué, but the article itself isn't). The author talks a bit about why the Narnia book covers have shifted in tone from psychadelic to grim to cutesy since their original publication:

Part of the trouble in coming up for covers for the books is that the Chronicles of Narnia are read by both children and adults now, as opposed to the '70s, when they were mostly read by the same teenagers and college students who had recently discovered the largely moribund and unpopular Lord of the Rings series. Because the Chronicles were decidedly easier for children to read by themselves, and contained absolutely no long ballads written by elves, the covers for the Narnia books have regressed quite a bit more than the Tolkien novels in terms of age targets, to the point where there is now a separate series of Narnia covers intended to render the books more palatable to adults.

(The LotR was 'moribund and unpopular'? Them's fightin' words!)

I grew up reading the Narnia books adorned with classic old-school covers, which I must confess do seem a bit goofy to the jaded modern eye.

(Hat tip: Looking Closer.)