These, you mean?

A quick search for "comic book reader" turned up this thread. It appears that at least three routes exist; as the .cbr format is just a rar, there's unrar; there's evince, which handles all sorts of documents, including cbr and cbz, and there's comix, which is designed for comics, but was said to be a little sluggish loading, in an early version... I quit skimming about page 3.