'The Right Stuff' by Tom Wolfe. Favourite book, favourite movie, one of the main reasons I ended up flying aeroplanes for a living (not quite being an astronaut but closer than your average office worker will get...)
Sphere by Michael Crichton. The film wasn't a GREAT film, but it's the nearest I've come to reading a book and the film being pretty close to how I imagined it.
Shawshank Redemption deffo. Started off as a short story in a book of short stories by Stephen King. Was originally called Rita Hayworth and the Shawshank Redemption.
'Wiseguy', by Nicholas Pilleggi, which became Goodfellas. OK, so it was based on real events, but a lot of films get those wrong too (see other thread).
Thought J.J. Connolly's Layer Cake was well filmed as well.