A good alternative is Introducing Spectrum Machine Code by Ian Sinclair. It provides an easy introduction to machine code but does not offer so much in terms of content as the Melbourne House issues.
Sinclair begins by introducing the reader to ROM and RAM. He shows how both those systems are used to give the user an easy way of programming. He then provides a long discussion of Z-80 programming. The book is sprinkled adequately with examples and many of the machine code routines explained are put into Basic listings in the form of DATA, to allow the user to enter them quickly. Introducing Spectrum Machine Code, from Granada Publishing, costs £7.95.