The Book of Margery Kempe, by Margery Kempe
This is the earliest surviving autobiography in English. Kempe lived about the time of Chaucer, 1373-1440.
Bede's The Ecclesiastical History of the English People
This classic early history is a book I've long wanted. I've read large portions online, but am glad to have a copy of my own.
It was a very "medieval" birthday!