It was written and first published in 1967 to help piston and turboprop pilots transition to jets, and that is the context in which it was written. And as all commercial jet pilots will at some stage be faced with this, it is still relevant. Sure, it's dated. It refers to the 747-100 as being a new aircraft. But as for handling and aerodynamics are concerened as the laws of physics haven't changed, even with the intriduction of glass cockpits and FMCs.
As mentioned, it won't help you pass the ATPL exams - it was never meant to. But it is excellent background reading and questions from the book still crop up in airline technical interviews - largely because the interviewers want to see how much background knowlegde you have. And guess where they got theirs from!
As for Davis' condescending manner. You have to remember he was the test pilot involved with the certification of most of the aircraft introduced to the British register from 1950 to 1970. His recommendations led to design changes - such as the stick nudger on the 747. He knows what he's talking about.
HTBJ is essential reading for all jet pilots IMHO.