This small reproduction doesn't do it justice, it's a full page illustration in the book.
Some things I like about it: these guys are not heroes, but they are serious, wary. The magic mouth is talking and they're paying attention, but not in awe of it, and not frightened by it. They've seen this kind of thing before. They have a goal and won't be denied.
Two side notes. All the years I've loved this pic, I never noticed the eyes in the darkness below, until today. And in searching for a digital image of this picture I found a post by Raggi, in which it seems we are in complete agreement on this picture.