So it's slow progress. I mean, my Chinese reading isn't quite up to the task, but I still feel like I'm getting somewhere. The Yanqing County Government website (should be a link in the blogroll under 'Beijing') seems to be all but useless- I can't open anything beyond the front page. But then I found this Baidu Baike article which seems at first glance to be potentially useful. Take the introduction as an example:
延庆县位于北京市 西北部。县域地处东经115°44′-116°34′，北纬40°16′-40°47′，东与怀柔相邻，南与昌平相联，西面和北面与河北省怀来、赤城接 壤。是一个北东南三面环山，西临官厅水库的小盆地，即延怀盆地，延庆位于盆地东部。总面积2000平方千米。2005年底户籍人口27.6万人。
Alright, it's not the history of the county, but still that's a lot more info than any of the other introductions to Yanqing I've come across, all neat and concise and ready to be used. I'll keep sifting through and see what I find about the history of Yanqing.
I may, perhaps, have bitten off a bit more than I can chew, but for the time being I'm enjoying this.