I just picked up Take Back Your Life by Sally McGhee, as a companion to GTD.
Total waste of money! (since you can download better and more succint info for $10.00 from this website, which I also did, after I bought TBYL)
However, here's the best thing I got from it so far.
For intraoffice email use prefixes in the subject line such as:
AR (Action Required)
RR (Response Requested)
RO (Read Only)
FYI (Information Only)
If everyone gets on board using the same system, this can make getting in to empty that much easier.