Working as I do in a rare book and manuscript library, I've seen some pretty wild books, but the 217 volumes in the Wooden Library in Alnarp, Sweden may take the cake. Each volume of this unique collection describes a certain species or variety of tree or shrub. The series was made in Nürnberg, Germany between 1805 and 1810.
Wooden libraries -- or xylothek -- from the Greek words for tree, xylon, and storing place, theke, flourished for a few years around the turn of the 19th century. They were a further elaboration of the cabinets of natural curiosities that were common during the 18th century, and consisted of simple pieces of wood specimens placed together in some kind of cupboard. In a refined form they took the shape of books where you could find details from the tree inside and arranged as a library.
Hat tip to Woods' Lot