Kurt Vonnegutt - Best writer ever, his best books are slaughterhouse 5 and Mother Night. Mother night is particularly dark and twisted.
Phillip K Dick - Brilliant sci-fi author, some of his short stories are the best. of course the brilliant A Scanner Darkly is one of the best.
Terry Pratchtt - YAY!! Everyone loves PTerry! keep fighting that Altzheimers TP. All his discworld books are wickedly funny, pick any and just read. It's usually best to pick one near the beginning of a story arc, but there's no need to know what happened in the last ones. Also, his 2 scifi books 'dark side of he sun' and 'Strata' are very good too. If you've read Ringworld, you should totally read Strata, it totaly nails RW if you know what I mean, very funny.
I used to like Michael Crichton's books. Jurrassic Park and Lost World were great, so was Congo but his new stuff is a bit shit. Especially State Of Fear. it's bloody awfull, disparate elements, shit characters, shit plot, it held together about as well as the story I wrote in year 8 English.
Oh my! I totally forgot, You Must By Law or you will be Beaten With Sticks go and read the His Dark Materials trillogy, by Phillip Pullman.
Best fantasy series ever. Go read Northern Lights (Golden Compass for you unfortunate yanks), it totally rules. If only they hadn'y fucked up the film
damn christian-appeasment.