The ComicSpectrum news Blog has been concentrating on the monthly sales numbers from Diamond for a while now. These are not perfect. They are sales to comic shops, not sales through to customers. This means a book that sells 200,000 copies to shops could have 150,000 of those copies sitting around unsold in back issue bins. The sales also don't take into account digital sales (accurate info on this isn't currently available publicly). It also does not cover sales outside of North America and the UK. But even though it isn't perfect, there is still valuable information that can be gleaned from the numbers.
What the data does show us are trends of what is selling to shops. A shop that has a habit of ordering book they cannot sell will typically go out of business and be removed from the data set in that fashion. If you look at the data for what it is instead of what you might wish it was, it gives some interesting insight into buying trends of comic fans in the US/Canada/UK.