The neat part about this is the amount of work that went into it. The hacker who made the xylophone had trouble finding books at first. Most didn't have a good sound, so he soaked the books in a glue solution and then a sugar water solution. I t didn't help any. Then he noticed that O'Reilly printed books on a different quality of paper, and he was able to use them as-is.
There's a fair amount of detail on his website (in Japanese), but what made it through Google Translate was mostly gibberish.