Goosebumps -- and it was this particular book.
My old sister read it to me when I was... five? four? Probably shouldn't have been, but I wanted her to read it to me. We didn't even finish the first story -- The House of No Return -- before I started crying.
Two years later, I'm in her room, reading through her goosebumps books.
These books were the start of my love for horror. And soon after, I started going to the library to borrow so many Goosebumps books. Then I moved on to R.L.Stine's Fear Street Series. Before long, I started reading Stephen King, H.P.Lovecraft, and a slew of other horror authors.
And here we are today XD
On a side note, I also loved the Richard Scarry Busy Towne Books and Marc Brown's Arthur series (and I would always try and see where Brown hid his name or his children's names XD).