I was lucky enough to be able to get a couple of books from the local library with coding examples. Usually these would be the usbourne books - which had BASIC listings for multiple home-computers at the time.

I did come across a couple of books that briefly documented assembly language stuff, and of course the (orange) manuals included with the computer contained a lot of good information.

Even now I have my Z80 books, they've moved house with me and even emigrated to a whole new country.