As with the Z80 you can't go wrong with an Osbourne & McGraw-Hill. All you need to know is clearly laid out and they always give lots of good code snippets.
Good starter book - particulalry comprehensive on the weighty addressing modes.
This was my starter book with the QL and although it doesn't go into the same detail as the previous two the teaching method is very good. He builds up a disassembler program in Superbasic as the chapters progress; there is also a listing of a simple assembler at the end.
Cover the language specific to the QL in more detail than the previous book, handy to have.