First I tried a Crosman Stinger spring-powered airsoft pistol, firing Crosman 6mm .12g plastic BBs. It penetrated only about a dozen pages, or less than 1/16th of an inch. The pellet clearly bounced off.
Then I tried a Marksman Repeater spring-powered BB pistol, firing Crosman Copperhead 4.5mm steel BBs. It penetrated only slightly farther, a little over 1/16th inch. The pellet clearly bounced off.
The third weapon I tried was a Crosman Phantom .177 spring-piston air rifle, firing Daisy QuickSilver pointed field pellets. This penetrated to a depth of 5/8ths of an inch, enough to kill most small varmints (gophers, pidgeons, rats, etc). I did not recover the pellet; it must have bounced out of the hole.
Finally I tried my Marlin 795 .22LR rifle, firing Remington high velocity 36gr plated hollow points. This blasted completely through the yellow pages and half-way through the white pages, a depth of over 650 pages, or about 1-7/8th inches. I was able to recover what was left of the bullet, but it was smashed beyond all recognition and fragmented into several pieces, some quite tiny.
I didn't dare try this with my .45 Colt or .30-06, as those would have blown completely through the phone book. I wanted to try various other combinations of ammo (flat-nosed pellets, darts, etc.) but it was too hot today.
Two fragments of the bullet I shot into the phone book, along with an intact .22LR round for comparison.