As with most medieval texts, the title of this autobiography is not part of the original manuscript. It was likely given its descriptive title when it was first printed in short form by Wynkyn de Worde in 1501.
As a title, The Book of Margery Kempe gives us the basics of what we need to know, but no more. All it really tells us is that this book has something to do with someone called Margery Kempe. We have to read on to find out why we should care.