The PNT trail is actually pretty good for biking, as a large fraction is dirt roads that connect smaller segments of trails (at least in WA). But good information is hard to find. I have the Northwest Trail Digest and find it mildly helpful in general information - but certainly well worth the $10. If you have time, I think it'd be best to just get out and explore a few sections on their own to get a feel for the areas.
Last weekend I did the PNT in western WA between Alger and Hamilton. It was fun, but doing this exact route in reverse would be otherworldly because of the single track that goes down east from Mount Josephine. I was happy to have gravity pull my bike down while it barely fit under / between fallen trees. It took me 45 min to get down 0.8 miles and 700 ft. At this particular point of the PNT the guide book only mentions "The trail continues for another 0.5 miles up the ridge toward Mount Josephine." While this is true, in terms of the effort involved to go up, this may well be a 2 hour ordeal with a loaded bikepacking bike. Pictures, GPX track and such:
http://2wheeltrails.wordpress.com/2013/10/22/bikepacking-the-pnt-from-alger-to-hamilton-wa/I have also ridden westwards to Blanchard Mountain and that area is a treat. Afterwards, there are lots of paved & dirt roads heading south-west to Deception Pass which I haven't explored, yet. I am sure it's beautiful when the flowers are out!