Here is the source code for the disassembler from Chapter 17 of Mastering Machine Code on your ZX Spectrum by Toni Baker as a zasm .asm file which will assemble to a .tap file.
It is available as a .zip file here which contains the disassembler .asm source file for zasm assembler, the assembled .tap file and an Xcode project file.
A PDF of Chapter 17 is available here. It is 11.6MB and based on images of a hard copy of the book kindly supplied by Adrian S. (cmonkey at World of Spectrum forums)
The OCRed PDF of Mastering Machine Code on your ZX Spectrum by Toni Baker is available here.
zasm assembler is available here
Known issues (from the included README file)
Z80 NEG opcode currently disassembles as “C HL,BC”. This will be debugged and fixed in a future update.
The README file also contains details of the edits required to the OCRed PDF as checked against the image based PDF.