The fur line reaches higher up your abdomen and you're pretty obviously a wolf now from the waist down. You finally build up the courage to reach out and touch the new appendage behind you and you flinch. At first it's too weird and you jerk your hand back, but soon you pull it around closer and stare at it intently, examining its pepper-gray fur with both hands.
"I have a tail now," you mumble only to yourself as if in a trance.
You decide it's too late to go out and find the book as your half naked at this point. You're less than half human now anyway, and you severely doubt anyone else in the library has seen anything like this before. You could yell, but even if there's anybody close enough to hear you, you'll be an animal by the time they get here. You might as well just sit back and observe the rest of your change.
You let go of your tail and look at the thick shaggy fur that's growing up your torso. It's a nice creamy color along your belly and fades into a dark gray around your sides closer t your back. You run your hands through it and moan: human hands against fur feels nice. It makes one of your wolf legs kick, and your new tail thwacks against the toilet again as it starts to wag on its own. You're feeling things only an animal would feel, and wolves are pretty cool animals; you aren't sure you don't want to be one.
The fur line reaches your shoulders and a new change makes you take your hands off yourself and hold your arms in front of you. They shorten until your elbows are barely outside of your chest. Meanwhile your hands grow longer even as the fingers on them shorten and thicken. Short fur grows down the whole length of them and lightens up toward the ends. Another set of pads grows in while your thumbs shorten into little dewclaws on the side. No more flipping through books for you for a while: you're going to be walking on all fours for as long as you remain a wolf. You're more than a bit nervous because you're not sure how long that's going to be. Surely an old library book couldn't take away your humanity for ever, could it?
The tingling rises up your neck as you wait for the magic to finish its job.