Secondary Steve Ciarca, author of "Build your own Z80 computer" (1981), so nice book.
Then the author of the VHDL version of yamaha Yamaha sound chip : [http://www.fpgaarcade.com/library.htm fpgaarcade]. And [http://opencores.org/project,t80 opencores] for the Z80 (T80)...
And websites that give access to so much old Amstrad resources like :