Transgender medieval studies is a growing field, so, my v. first piece of advice is: keep checking. There are going to be several panels devoted to this at next year’s international medieval congress at Kalamazoo (this is a real thing that exists, I swear) and that’s pretty reliably live-tweeted. One of the greatest feminist medievalists issued a call to arms for more uncovering of trans* histories at a conference I was at last month, so… we can expect more of that. This online index for the history of women and gender has a good list of search results for “transgender.”
As for perceptions, performance, etc. of gender in the Middle Ages: this, on gender and difference, is good; I love this collection of essays on the body. You can probably read most of the intro to this other collection of essays on Google Books. The Passion of Perpetua and Felicity, where Perpetua dreams herself as a man, might be helpful. The lives of saints and mystics are generally good source material, as in these writings (especially the latter) gender gets REALLY fluid, really fast. There’s been discussion of how clergy and vowed religious persons might form a “third gender,” as well. This review gives a good overview of some of the history of studies of medieval gender.
There’s a monograph arguing that medieval romance literature does not portray bodies/beauty in gender-specific ways, rather rejecting gender dimorphism… can I find the title of this? Annoyingly, no. I remember feeling that it pressed its argument too far, ignoring evidence of how embodied/perceived gender affected people’s lives.
On a separate but potentially related topic, this lecture is mostly on sexual orientation and on some of the challenges of applying this idea to study of the Middle Ages. I love this book on sex and sexuality, and use it in teaching. This blog devoted to a progressive/activist practice of medieval history has a number of relevant posts.
ETA: I can’t believe I forgot the Middle Ages’ most prominent transwoman: Jesus. Stay with me here. It was quite common to imagine God (and esp. Jesus) as a woman. See this classic collection of essays; these 14th-century writings by Julian of Norwich. Also (though I can’t currently find my reference for this) devotional images of Jesus’ wounds, made in women’s convents, often resemble the vulva. Catherine of Siena and others also often describe Jesus as giving birth. It’s complicated and could be a discussion of its own, so I’ll stop here.
