A B757-200 First Officer reported a trailing edge flap problem on approach. The flight crew declared an emergency and diverted to a more suitable airport. First Officer commented that the procedure was complicated by a new; poorly written QRH.

Narrative

I was hand flying; raw data; the airplane [was] in VMC on a long base leg cleared direct to the airport. Descending through 11;000 FT; I called for flaps 25 and noticed the TE flap disagree light. The flaps were stuck at 20 with the flap lever at 25. We notified ATC and told them we had a flight control problem and needed to break off the approach. They gave us a climbing right turn to 15;000 FT; heading 010 to head north through the valley. On the climb out we moved the flap lever to 20 and raised the gear. After level off; we moved the flap lever to 15 to see if the flaps would respond; but they stayed at flaps 20. We declared an emergency and decided [a nearby airport] was our best option. Told ATC we wanted vectors and started heading that direction since fuel was an issue and we didn't want to land at [original destination] with a flap problem. Per the checklist; once we engaged the ALTN system; the flaps moved to position 15; and the TE disagree light extinguished. Then it had us; using the ALTN system; lower them to 20; our landing flap setting per the checklist. We tried to reach Dispatch; but had no luck. We asked ATC to contact our Dispatch. It took several attempts and close to our divert field before we were able to reach Dispatch and Maintenance. Since we were both unfamiliar with the QRH and have found multiple errors and omissions; we decided to back each other up and both look at the QRH once the autopilot and auto-throttles were engaged. This was a very unnerving feeling since there was no real division of duty and some loss of situational awareness. Luckily; we talked about one runway operations and added 2;000 LBS of gas. We did a flaps 20 landing and touched down on the numbers. We brought it to a stop at the end of the 9;000 FT runway with an uneventful outcome.