I've heard some of the guys in the magazines talking about how a bullet "performs" when you shoot it into a wet phonebook (or phonebooks)
Does it somehow represent how a bullet will perform on an animal?
If you guys think this is worth while, how does it work? What yardage etc...
Or is it a waste of time and ammo?

no right or wrong opinions here.....IMO wet phone books are simply a bad way to look up someone's phone number.

Bullet performance on an animal involves hide, lung material, bone. muscle, and any number of combinations of these. No medium is the same.

Probably geletin is as good as any for approximating performance. However wet paper is better than shooting thru a long series of pine boards.

I have used the wet phone books for about 30 years to get an idea of what to expect from a bullet in terms of terminal performance. You bundle them with twine in stacks about 10" thick, and soak them in water for 24 hours. They swell as they soak up and it usually takes two stacks to stop an expanding bullet. The general "benchmark" for this type of testing is a common round, like the 30/06 Remington 180 gr. Core Loct. This bullet usually will penetrate about 12" in the wet phone books and make a certain shape "wound channel". You can then compare other bullets and , knowing from field experience what the 180 Remington Core Loct will/won't do, make projections about other bullets. Not a substitute for actual field testing, for sure; but a good indicator of what to expect.

NO MATERIAL has been found that accurately simulates how a bullet will act when it hits a real live critter! Ballistic gelatin may be close, but it has no bones!!

A wet telephone book, or any other wet anything, can only allow you to compare how different bullets act when they hit a wet whatever - and this means you have to get each whatever just exactly the same wetness as the last one you shot into to make the comparison valid-a very difficult thing to do.

The best way to determine bullet performance on game is to trace out the wound channels in actual game animals-an autopsy, if you will! Damn few people ever bother to do this thoropughly enough to come to valid conclusions.

The most recent comprehensive work along these lines that I am aware of being in print is in Bob Hagel's book, GAME LOADS AND PRACTICAL BALLISTICS FOR THE AMERICAN HUNTER. This is a book well worth reading.

Wet phone book, 2"x6" pine boardand 1gal of water/dye, bunched together.
If it goes thru all @ 100yds, you have nothing to worry about the penetration.