As we should already know: in machine learning it is essential to evaluate a model on data that it was not learned from.
in Dan Jurafsky and James H. Martin “Speech and language processing: an introduction to natural language processing, computational linguistics, and speech recognition”, 2009, Pearson Prentice Hall, Upper Saddle River, N.J., Second edition, ISBN 0135041961
This is the new version. Still under construction.