Not that I'm looking for anything but spiritual sustenance from any book, but several on my shelves are deemed valuable by booksellers. It did surprise me that the most dollar-valuable book I own is Plain Song, Harrison's first book of poems, published by Norton in 1965.
I would have thought the most sought after would be his third book, Letters to Yesenin, because (according to what I read one time) Norton declined to publish it, so it was put out by a small press located near Harrison's northern Michigan home -- probably a press set up just to publish this book and owned or controlled by the poet himself.
His third book, Locations, also published by Norton, I have only in a paperback edition. The poems within are equally as excellent as they are in the hardback edition.