Ron wondered this too, in *Harry Potter and the Deathly Hallows*:

>"This is the one that gives explicit instructions on how to make a Horcrux. *Secrets of the Darkest Art* - it's a horrible book, really awful, full of evil magic. I wonder when Dumbledore removed it from the library... If he didn't do it until he became headmaster, I bet Voldemort got all the instruction he needed from here."  
"Why did he need to ask Slughorn how to make a Horcrux, then, if he'd already read that?" asked Ron.  
"He only approached Slughorn to find out what would happen if you split your soul into seven," said Harry. "Dumbledore was sure Riddle already knew how to make a Horcrux by the time he asked Slughorn about them. I think you're right, Hermione, that could easily have been where he got the information."    
<sub>*Harry Potter and the Deathly Hallows,* chapter 6</sub>