From Atlas Obscura:
A xylothek (from the Greek for tree, xylon, and storing place, theke) is an object where the container is a fundamental component of the contents. The term usually refers to books that are both made of wood and filled with wood specimens. Xylotheks (also spelled xylotheques) first began appearing at the end of the 17th century in cabinets of curiosity. As time progressed, they grew larger and more systematic, with hundreds of individual volumes in a single collection, and are now consulted by those working in forestry, botany, forensics, art restoration, and other fields.
Xylotheks were particularly popular in late 18th century and early 19th century Germany. In these constructions, each book in the xylothek was made out of a particular type of wood, the spine covered with the corresponding bark and decorated with associated moss and lichens. Once opened, the book would reveal samples of dried leaves, flowers, seedlings, roots, and branches, with a special compartment in the spine holding a written description of the species’ biology and use. The Special Collections department of the Swedish University of Agricultural Sciences library in Alnarp, Sweden, contains a beautiful example of this type of xylothek, made in Nürnberg, Germany, at the start of the 19th century. Similar xylotheks are also found in France, Austria, Italy, and the Czech Republic.
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Today the term xylothek is also used to refer to huge collections of wood specimens not necessarily housed in wooden books (like those at the Samuel James Record Collection of Yale’s Forestry School, which contains 60,000 samples). But the origins of such collections lie in these more humble and yet strikingly beautiful objects, which have been made all the more precious as deforestation renders some trees extinct.
Link to the rest at Atlas Obscura and thanks to Maggie for the tip.