So I just found out why I've had such a hard time finding the Dewey decimal numbers for books: they aren't free!
A company actually makes money off of telling you what the Dewey decimal number for a certain book is!!
The good news is, this company will give you the first few decimals for free.
Other good news is that you can search the book and see if it's already been assigned a Dewey decimal (some databases list this).
There's more details here.
Ahh... another mystery evaporates...