Existence of the threat book has been widely known since the early days of the Cold War. The contents come from a variety of sources, none of which I'll mention here, and is compiled in final form by a committee. Some presidents read it carefully, eg: JFK, Bush 41 and, during the dust-up in the former Yugosolvia, Clinton. Other presidents have merely asked the briefing officer if it contained anything especially of interest that wasn't going to be in the later DOD brief-- LBJ. Nixon and Reagan gave it cursory look and--I don't have any idea what GWB does.
It is my understanding that GWB receives a Threat Matrix debriefing nearly each and every early morning since 911.