Or early modern English of the mid to late 15th century?
I suggest for late medieval, as Matt did by grabbing some Chaucer, followed by the Pastons, Stonors and the Celys, all three families have surviving letters and the first two predate the 15thc, showing how language changed quite quickly over a hundred years or so. It depends on which part of the middle Ages you are needing. A good library will have all three (five in reality as there are three Paston Letters and papers)
Pastons online, most of them are herehttp://etext.lib.virginia.edu/toc/moden ... sLett.html
Paston Letters and Papers of the Fifteenth Century: Part I: Pt. 1 (Early English Text Society Supplementary Series), ISBN-10: 0198124155
Paston Letters and Papers of the Fifteenth Century: Part 2: Pt. 2 (Early English Text Society Supplementary Series), ISBN-10: 0197224229
Paston Letters and Papers of the Fifteenth Century: Part III: Pt. 3, ISBN-10: 0197224237
ISBN-13: 978-0197224236 (Early English Text Society Supplementary Series) (not got this one, it is on my xmas pressy list)
Kingsford's Stonor Letters and Papers 12901483 (Camden Classic Reprints), ISBN-10: 0521555868, ISBN-13: 978-0521555869
The Cely Letters 1472-1488 (Early English Text Society Original Series), ISBN-10: 0197222757, ISBN-13: 978-0197222751
All of the above have photos of some of the letters, the Pastons have a large selection, all referenced to their appropriate collection.