I have several books that have Giger's work in them (Necronomicon I + II, Giger's Alien, Species Design by Giger) and my feelings are mixed because of the price for the Necronomicons. I bought them originally for $40 each, which I would say is absolutely worth it. At the $280 it's currently listed for on Amazon? I think that's a stretch. The size is fantastic (about 16" high by 12" wide) but their length is relatively short (fewer than 100 pages in each). If you're a Giger fanatic and have the cash to burn, the work in these is fantastic and larger than any other books I know of.

Taschen put out a larger book of Giger's work (Biomechanical World) which is also beautiful and relatively exhaustive. If you want to see more of his work, I'd recommend that one. If you're more interested in his work for Alien, I'd recommend Giger's Alien or The Making of Alien.