I recently picked up a couple of awesome art books off eBay. The art of Adrian Smith and the art of Clint Langley. Have you heard of Langley? The guys a genius. I love his work. He also does some Slaine comic work that is very dark and visceral. He incorporates paintwork with real life imagery through digital manipulation. Cool stuff man.
You can stumble upon the
40k art books I mentioned on various second hand websites. But they're not cheap usually. I was lucky enough to find mint copies at a steal so I jumped on em.
There's also a big hardback book called the art of warhammer
40k. I meant to buy it once, but again not cheap and I don't really know if the contents are any good.
Google is your friend.
Happy hunting