Someone on a list I belong to mentioned that Agnes Strickland's Lives of the Queens of England was downloadable from Google Books, and sure 'nuf, it is--at least one volume, anyway. Strickland isn't always reliable, but her accounts are a good jumping-off place. One hopes that soon more volumes will follow.
I also was excited to see (OK, I excite easily) to find that I could download a volume of Lives of the Princesses of England by Mary Anne Everett Green. Green is quite useful, even today. For those of you who like really old England, there's Lives of the Queens of England before the Norman Conquest by Matthew Hall; how reliable he is I have no idea.
Some volumes of the Close and Patent Rolls are also downloadable now. There's lots more, depending on your taste.
There's been some controversy about Google Books, as you probably know, but I for one think it's wonderful that these public-domain books are being made available to the public. The downside? You'll spend too much time looking through the things when you should be doing something else. I, for instance, got sidetracked by a Emily Sarah Holt novel called The Well in the Desert, which isn't available for download yet but which can be read in full. I didn't read the whole thing, but it evidently deals with the annulment by Richard Fitzalan of his marriage to Isabel le Despenser in the 1340's. I always thought Fitzalan was a cad, and Holt certainly does, making him a wife-beater to boot.
Off to do some more Googling.