This LISNEWS post showing a picture of a Christmas tree made of books sparked another library to do the same thing. They used their National Union Catalog books to build a tree. You can see the results here. Another photo showing the building in progress.
With their pictures of the tree building the William H. Hannon librarians also linked to this piece: What are all those green books on Level 3? The piece talks about the National Union Catalog. (The 754 volume, 528,000 page, 13 million+ record National Union Catalog, also known as "Mansell" (after its publisher), took 14 years and $34 million to come to fruition.)
For new librarians that have known nothing but electronic catalogs this piece sheds light on important history in librarianship. The article also has this interesting line:
Although our online catalog and resources like WorldCat have mostly rendered Mansell and others like it obsolete, it can still be a useful tool for researchers needing exhaustive searches for pre-1956 works and for catalogers of rare books. In fact, a 2005 study indicated that 27% of the records in Mansell still cannot be found in WorldCat.