A book on machine code, Spectrum Machine Code, has been published by Shiva. The authors are Ian Stewart and Robin Jones and their handling of the subject is excellent, so far as it goes.
The book is very thin, although it provides a great deal of information. The chapters on the display and attribute files of the Spectrum are very interesting and will be of immense value to anyone who wants to use machine code graphics within programs.
Spectrum Machine Code is part of the Shiva Friendly Micro series and costs £5.25.