CMRLS just received a copy of Learn Dewey Decimal Classification (edition 22) by Mary Mortimer. It is now cataloged (in Dewey, of course) and listed in C/W MARS.
This is a great workbook for understanding how DDC works. It goes into necessary detail of the different parts of the 4 volume set explaining how each section works and how to use them together. There are lots of excellent exercises.
We also own the earlier edition and will keep that in our collection. Even though some of the numbers are different (I actually haven't comared the two editions side-by-side), the process is still the same.
I'll be using this book in my upcoming Cataloging Workshops, so it is not available for circulation until after April 7, but please take a look at it when you can. It will likely answer all of those Dewey-related questions you have.