I bought the GTD book. But then I went on to misplace it and didn't find it for over a year. I might be a hopeless case.
I have gotten good results from calendar blocking, so obviously I disagree with Davids rule about only putting things in the calendar that has to be done then. I can see his point for a physical calendar, but on a digital calendar you can have a (say) red calendar that your "must be done at this time or not at all" appointments can be added. That way when you see a red calendar entry you known what it means.
This was not really practical in Davids time.