The book cover is off to a great start. I hope you don't mind but I did a paint over. I didn't have a lot of time so I did what I could to help. Please take my opinion with a grain of salt. Either way, I hope there is something here for your.
I felt the whole image was a little to uniform. The first thing I did was add a consistent lighting source. Seeing as how this images is a collage and scale/space is not unified, it needed something to act as the glue which would hold the composition together. I choose the lighting and mass value pattern to do so. This is common practice. Next, I added a glowy bit to the primary character's chest so the lighting made sense... give or take. Obviously I took some artistic liberties with the lighting cues but for the most part, light is on the light side and shadow on the shadow side of the characters faces. I then melded some shapes for the graphic elements of the image. It seemed appropriate that everything but the faces would take on more of a graphic nature so I did just that. I also changed the scale and reorganized some of the heads to feel closer in scale for reliability. They felt too sporadically placed in space. I then knocked all the lighter values from the setting that were competing with the faces. This gave the faces center stage which I felt was appropriate. I also added what could be dragon heads to the left and right of the dude's head at the top center. I took out a lot of the scribblie strokes when I blocked in the light and shadow shapes. I took them out of the mass value shapes as well. I didn't feel the noodling added anything to the image but messiness.
Now, If you squint your eyes at the image you will notice the underlying circular clock like structure. This is the first subconscious read a viewer will make. My goal was to keep the eyes coming back for more. And seeing as how this is essentially a circular composition, bringing all the light shapes into the circle made sense to me. Almost all choices were made to reinforce a strong clear composition and pleasing aesthetic's through the use of simple picture structure.
Other things of note: Most of the faces felt off. They lacked consistent facial structure. I didn't have time to fix everything but I hope you get the idea. Stay consistent with the planes of the face. The eyes were tilted one way and the mouth another and yet the nose another. Also, one half of the faces looked different from the other like they were two different faces grafted together. Staying consistent with the symmetry is really important to a good consistent facial structure.
And finally, I changed the facial expression to reinforce a consistent emotion. I moved the gunner's arm and fixed the hand's proportions too.
Anywho, here it is.
And an animated gif to show the difference; before and after.
Again, I hope this helped. You are off to a great start. I can't wait to see the result.