Thrillers:
The John Sandford "Prey" series, starting with "Rules of Prey " and going to the most recent "Broken Prey" - Lucas Davenport is an interesting series character, and the series is a lot of fun. Read them in order though, Davenport goes through some changes throughout the series.
Sci-FI:
1) Stand on Zanzibar, by John Brunner - Hard to find, but it is a masterpiece of dystopian SF
2) Ilium, by Dan Simmons - Troy recast as SF; fascinating and superbly done.
3) Stranger in a Strange Land, by Robert Heinlein - You may love it, you may hate it, but it's the most important SF Novel of the last 50 years
4) The Forever War, by Joe Haldeman - If Tim O'Brien wrote SF, he'd have written this. Angry , moving, eloquent, it's a grunt's eye view of galactic conflict.
5) The Mote in God's Eye, by Larry Niven & Jerry Pournelle - The GREAT first alien contact story
Mystery:
1) anything by Dashiell Hammet, Raymond Chandler, or P.D. James
Horror:
Carrion Comfort by Dan Simmons - psychic vampires and fabulous
Song of Kali by Dan Simmons - The "Heart of Darkness" of Horror novels - Harlan Ellison called it the greatest first novel ever written.