That is an excellent book, and the best introduction to text crit I know.

Erhman, it's important to note, isn't questioning the attribution of these books to the various people they are traditionally attributed to. I mean, he rejects those attributions, except for some of Paul's letters, but scholars have known Luke didn't really write Luke for ages.

What he's trying to get at is how we know the text is secure, i.e. those words are what whoever wrote them really are the words they wrote. That's called textual criticism, and I've written about it several times. Here is the entry in the FAQ.

There's more to say, of course, but that should get you started.