Evan's father has been transferred to Atlanta, so Evan has to stay with his great-aunt Kathryn, while his parents look for a new house. His great-aunt is completely deaf, refuses to learn sign language or to lip-read, and is, in his father's words, a bit weird.
It's not all bad, though. When he takes his dog, Trigger, for a walk, he meets Andrea--Andy. She's twelve, like him, and they share a sense of humor, so they become fast friends. While looking around a toy store, they find a can of 'Monster Blood.' It's cold, stretchy, green, bouncy, glows in the dark, and seems to be growing larger. After trigger eats some, he grows bigger, too. Soon, Evan has more Monster Blood than he can handle, and seems almost... hungry.
Monster Blood is the third entry in R. L. Stine's Goosebumps series, and is reminiscent of The Blob. It must have proved popular, because it was followed up by several sequels, including the sixty-second and final Goosebumps book, Monster Blood IV.
When Evan first gets a look at the Monster Blood in the can, he's a little disappointed. He's had other stuff like it before, he says, called 'Alien Stuff' or 'Yucky Glop,' or something like that. He's in no hurry to be rid of it, though, and he and Andy play around with it, tossing balls of it back and forth. One goes astray, and, before they can stop him, Trigger eats it. Evan is worried that the Monster Blood might be poisonous, but Trigger doesn't show any ill effects, at least at first.
Soon, though, Trigger begins to grow. He outgrows his collar, first, and soon he's nearly doubled in size. Evan takes him to a vet, who runs some tests, but assures Evan that Trigger seems to be very healthy, if not a little large. He promises to get back to Evan when the blood test results come back, so Evan leaves. Amid all this, two bullies--twins--called Rick and Tony Beymer have decided that Evan is their new favorite punching bag, harassing him and eventually beating him up.
Eventually, the Monster Blood has grown so that it can nearly fill a bathtub, and Evan and Andy decide they'll have to take it back to the toy store they bought it from. When they arrive, though, the toy store has closed, and they're left with no options.
Finally, they try sealing it in a garbage can at Evan's great-aunt Kathryn's house, but it escapes and begins to chase Evan and Andy. The Beymer twins, who'd stumbled across Andy, were swallowed up by the Monster Blood, and when great-aunt Kathryn opens her door, it chases her inside, too. When Evan and Andy race inside, they find Kathryn, about to be swallowed by the Monster Blood, insisting that she'd created it, and so she must die for it.
Unexpectedly, Kathryn's black cat, Sarabeth, turns into a young woman--a witch, as it turns out. For twenty years, she'd made Kathryn her slave, making her deaf and forbidding her to read lips or learn sign language. When Evan came, she was afraid that she'd be revealed, so she forced Kathryn to cast a spell on the Monster Blood in order to rid herself of him.
With the help of the now pony-sized Trigger, Sarabeth is forced into the Monster Blood, devoured by her own creation. At this, the Monster Blood begins to shrink, and Trigger returns to normal size, too. Great-aunt Kathryn can hear again, Sarabeth's spell on her being broken, and even the Beymer twins escape any harm more serious than a good scare. When they look, though, the Monster Blood is nowhere to be found...
Monster Blood is fairly entertaining, and the twist at the end is better integrated into the story than in the previous two Goosebumps books. I think that the prose is better in Monster Blood, too. The story of an all-consuming blob is a fun bit of classic horror, repackaged for kids. This one's also available for the Kindle, as Classic Goosebumps #3, so give it a try, whether you prefer paper or electronic books.