Goosebumps - Night of the Living Dummy - R.L. Stine

Paperback, 

When twins Lindy and Kris find a ventriloquist's dummy in a Dumpster, Lindy decides to "rescue" it, and she names it Slappy. But Kris is green with envy. It's not fair. Why does Lindy get to have all the fun and all the attention? Kris decides to get a dummy of her own. She'll show Lindy. Then weird things begin to happen. Nasty things. Evil things. It can't be the dummy causing all the trouble, Can it?