The Fall of Reach by Eric Nylund
yes, it's a Halo book, but don't let that fool or deter you. once you've read it you'll soon be flipping through the pages of First Strike and Contact Harvest as well. ;)
here, a random section from First Strike:
After the Chief had left to investigate the cryopod, Haverson made certain that the bridge doors were locked. He turned and walked over to the Covenant Engineer who'd repaired the Master Chief's armor.
"Fascinating creatures," he murmured. He drew his sidearm and pointed it at the back of its head.
Two of the Engineer's six eyes locked onto the muzzle of the weapon. A tentacle reached for it, split into fine probing threads, and touched the blue-grey metal. Cortana asked, "what are you..." Haverson shot the Engineer. The round tore through its head and spattered gore across the display the alien had been repairing. "Haverson!" Cortana cried.
sure, not a very dense example, but the writer has a B.Sc. in chemistry and a M.Sc. in chemical physics, so certain parts are above average "hard."