You will not find here the actual "Undocumented DOS" book by Andrew
Schulman et all, sorry. You will have to spend some money to get it
from a used books library (the book is worth it!).
You can, however, download the content of the floppy disks that were
attached to this book, in case your 5.25" floppies went bad.
BIN Undocumented DOS 1st edition.zip
BIN Undocumented DOS 2nd edition.zip
If you're specifically interested in the PHANTOM.C program distributed with
the 2nd edition of the book, you might want to take a look at the improved
version published by the OS/2 Museum in 2012 and named "PHANTOM 3.0":
TEXT Article (phantom30.txt)
BIN Program (phantom30.zip)