I've been meaning to mention the New York Times' songwriting blog for a few weeks. As someone interested in both the meaning of pop songs and how they mean, I find the writing here satisfying and enlightening in a way that very little published musical criticism is.
It's nice (but not necessary) that the authors (Andrew Bird, Darell Brown, Rosanne Cash and Suzanne Vega) are also well-known musicians. In fact, musicians' views of their own songs are often a distraction that keeps you from paying attention to what the song means to you.
But these musicians are very focused on the often stumbly process of song making and how even for the creator, the meaning of the song emerges from the process and the song itself. They address the sweet spot of music appreciation that sits between the technical and the emotional and try to show how a note hit a little flat or the acoustics of a room can illuminate a whole song.
Another example of this deeply in-between way of thinking about pop music is Tim Riley's book about every one of the Beatles' songs. I wonder if you could write a similar book about ads.