Narnia also introduced me to fantasy. Eventhough it's not my favorite genre, it's the only genre where I have two complete series (Trylle trilogy is the other one)
The Horse and His Boy... Finding that happens while the Pevensie are in Narnia as kings got me like o.o I wasn't expecting that.
For the covers, I noticed on goodreads that are other versions, all the places I've seen the books here in Venezuela have covers with the same style as The Silver Chair.