In reply to EarlyBird:
I've climbed several unclimbed peaks and made new routes on others, in Greenland and elsewhere. Often, a chance photo or conversation has prompted an area to go to. If there is no guidebook, then the American Alpine Journal is probably your best starting point, followed by the Alpine Journal. Having sorted through all the expedition reports to see what expeditions have already been done then you need to try and track down the full expedition reports. The only way to really do this, unless you own a complete set of AAJs and AJs, or know someone who does, is to spend a day in the Alpine Club library in London (which has full sets of virtually everything) followed by a day in the RGS, also in London where most of the UK expedition reports are held. You may need to write abroad for foreign reports. The situation is slightly muddied by some commercial expeditions which often do not file reports. Generally these tend to go for less technical targets though, so anything reasonably challenging is likely to be documented.
If you are in the AC you can consult both libraries free, otherwise there is a small fee.
My last Greenland report is online and includes a few ideas for other expeditions to go at: https://www.needlesports.com/needlesports/greenland/staunings2003report.pdf . However, as with the alps, climatic conditions are changing and this may affect the best time of year to go and the best method of transport. Also places that did not used to require permits, may do so now and places that did require permits may require different ones. So, old expedition reports are good to know what has been done and good for ideas, but not so reliable for planning if they are more than a few years out of date.