I'll second Hitchhikers Guide for a good sci fi recommendation. Another fun easy sci fi read is Enders Game. For something with a little bit more meat to it, check out Childhoods End by Arthur C. Clarke.
For fantasy, check out The Golden Compass by Philip Pullman. The trilogy is one of my favorite stories of all time.
For horror, I say check out Dean Koontz. People like to throw shade at him as a wannabe Stephen King but he has creeped me out plenty in his stories. They are easy to read too. I'd suggest either Phantoms, or Fear Nothing.
Lastly, I suggest checking out the Jack Reacher series by Lee Child. They are better represented by the Robert Downy Jr Sherlock movies than by the actual Jack Reacher movies in my opinion. If you like the fight scene break downs and the clues/mystery aspects, you'll dig Jack Reacher I think.