The Alien and Alien³ novels are excellent and well worth getting the omnibus for. The Aliens novel is kind'a weak, not because of any failing on ADF's part, but because the publisher made him tone the swearing down -- "Get away from her you bitch" is now "Get away from her, you." But they're all well-written and the first and third novel add quite a bit of neat details. I'd say Alien's adaptation is the best of the lot.