As previously noted, the call number for "The Oxford Companion to Musical Instruments" by Anthony Baines is 784.1903. Why is that? How do you come up with a Dewey classification number for a book? This one is actually pretty straightforward. It is built by combining 784.19 Instruments with the standard notation 030 Dictionary. Since no classification number is allowed to end in a 0 we end up with 784.1903. Of course this one was easy since I already knew the answer.