More than once, I've gotten that "ready for pickup" notice and the book was nowhere to be found. I've gone back several times, still nothing. And the library employees could never track it down, either. One time I was told "just because you get the email that says it's ready for pickup, that doesn't necessarily mean it's
ready for pickup... come back in a few days."
The branch across from the main library (Mid-Manhattan), on 40th and 5th, seems to be the worst offender, in my experience. I gave up on it.