Much as I adore books, there aren't many I'd consider reference specifically for playing Thorn. He has driven me to a renewed interest in poetry, but I would have to say that much of the knowledge of the late Middle Ages I've drawn on to play him is accumulated over time, and his somewhat fantasy-world setting has allowed me some leeway for historical accuracy.
Five books that influence Thorn:
Poem a Day, Volume 2 (Various)
This is, I believe, the only book that I purchased with Thorn in mind. Many of the poems used for calligraphic gifts from Thorn have been drawn from here.
The Medieval Village (G. G. Coulton)
This reference is around the house, I'm unsure to whom it belongs, but it's come in useful for reference to the feudal system.
Poemcrazy: Freeing your life with words (Susan G. Wooldridge)
This was a gift from a friend, but has been a helpful guide in my own attempts to write poetry from Thorn's voice... which was originally for the Nexus 100, which I have yet to complete.
The Calligrapher’s Studio (Claire Secrett)
This is one of those listed in my LibraryThing account as 'not really a book', but there is a sort of manual with it, with helpful tips on the art of calligraphy.
A Vision of Light (Judith Merkle Riley)
I no longer own this, and was startled when I looked it up to discover that it's a trilogy now. The copy I had was a heavy hardcover with a lovely almost metallic dustjacket. Excuse me while I sigh in nostalgia a moment. The lead character and storyline have nothing to do with Thorn, but it is the most detailed description of daily medieval life I've ever seen.