I also bought "The Book of Alien", which has been in print for a long time. I used to own a copy years ago, but it was borrowed and never returned. There is a page or two in it devoted to (unused) Space Jockey hieroglyphs as designed by H. Giger. They show the Xenomorph reproductive cycle in precise detail from egg to chest-bursting. This would seem to indicate that the Engineers were indeed building towards this perfect organism, and the weapons base we discover together in 'Prometheus' uses either an archaic or further refined version of the organism. I would suggest its the former, mostly because the bio-mechanical design of the ship and Space Jockey from "Alien" seems much further advanced than the technology in "Prometheus".