Three semi-loose thumbnails for each book.
Tighter renderings of the two thumbnails I've decided to proceed with. I posed some Daz3D models for reference on the figures and looked at several different deer-like animals for the unicorn.
I made this little maquette of the giraffe-like alien for the Out of the Silent Planet cover. I made sure to make the limbs free-moving so I could use one maquette for all of the different poses the creature appears in. To make sure that I got the right angle in all of the photos, I created a reference point for the horizon line. I made a blob out of foil and tape that is a stand-in for the human character that appears on the cover. The blob is scale-wise the correct height that the character should be in comparison to the giraffes. When I took my photos, I made sure the "horizon line" (in this case, the end of a ironing board covered in green towel) ran through the lower half of the blob, just like it does through the character in the thumbnail. This trick would not work so well with a higher horizon line since the end of the ironing board would be visible and not represent the true vanishing point.
After taking my reference photos for the creature, I combined them into one image to continue working out the composition.
Using the previous photo and several others for more detailed reference, I created the current composition with yet more changes. Most striking is the greater presence of the tall pointy things or Malacandrian "mountains" in the background. With a tight drawing in place, I can move on to rendering with a very clear direction.