It depends exactly on the rights on the book.
Usually it will permit you to copy small amounts (eg for citing purposes),
but copy will stop working after a time (16 copies or 16 pages rings a bell, but not sure what the rules are exactly)
ctrl-I while reading the book will give a little more info.
However, if it says 'copying not allowed' it is not quite as stringent as that suggests,
you can copy to other devices authorized with the same ID, and should be able to do limited ctrl-C copy as above.
Ok, then this book does not allow to copy a single letter...
Thank you for your answer.