I have also made a list of books on the 68000 (used for the ti92), you can find it here. Unlike Z80 books 68000 books should be very easy to get because they used in so many computer like Apple and Amiga. I find it easier to read books which only discripes the processor and not a whole system, so my advice would be to get books and the microprocessor and not Amiga/Apple.

Books in english

Title	Author	Year	Comment	Publisher
Programming in Z80 assembly language	Hutty, Roger	1984		Macmillan
The 8080, 8085, and Z80: chardware, software, programming, interfacing, and troubleshooting	LaLond, David	1988		Prentice-Hall
An introduction to Z80 machine code	Penfold, R. A.	1984		Bernard Babani
Introducing Z-80 assembly language programming	Sinclair, Ian	1983		Butterworth
Z-80 - 1979-80, Book 2: Programming and interfacing	Nichols, Elizabeth A.	1979		Howard W. Sams
A Z-80 workshop manual	Parr., E. A.	1983		Babani
The 8080/Z80 assembly language : techniques for improved programming	Miller, Alan R.	1981		Wilev. cop.
Z80 assembly language subrutines	Leventhal, Lance	1983	Contains many very good subrutines for math, string, memory and a lot more, but not a good introduction.	Osborne/McGraw-HIll Cop.
101 projects for the Z80	Tedeschi, Frank P.	1983		TAB Books cop.
Z80 instant programs : machine code rutines for Nascom and other Z80 computer systems	Hooton, J.	1982		Sioma Technical Press
Microcomputer technology : an introduction	Ullman, Julian R.	1982		Pitman
Z80 users manual	Carr., Joseph J.	1980		Reston Publishing cop.
Z80 and 8080 assembly language programming	Soracklen, Kathe	1981		Hayden
Programming the Z80	Zaks, Rodnay	1979+1980+1981	Read choice for beginners. Read this to understand the basics of Z80 assembler.	Sybex
Z80 assembly language programming	Laventhal, Lance A.	1979		Osborne/McGraw-Hill
Z80 microcomputer design projects	Barden, William	1980		Sams
The Z80 microprocessor : Architecture, interfacing, programming, and design	Daonkar, R.S.	1993		Macmillan
SGS-Thomson Z80 microprocessor family : programming		1990		Kista
SGS-Thomson Programming reference Z80		1990		Kista
Z80 applications	Coffron, J. W.	1983		Sybex
Z80 PIO, Z80A PIO. Technical manual		1978		Zilog
Z80-CPU Z80A-CPU. Tecnical manual		1977		Zilog
Z80 Programmers Reference				Zilog
Build your own Z80 computer. Design guidelines and application notes	Ciarcia, S.	1981		Byte Books
Spectrum Machine Language for the Absolute Beginner	William Tang		It's a great book and has a lot of non specific Z80 stuff. Just ignore the Spectrum only bits as sound and the game.
TLCS-Z80,85 Data Book				Toshiba cop.
Z80 Assembly-Language Programming	Lampton, Christopher		It refers to the old TRS-80 and other CP/M computers, but it can be generalized and applied to the TI fairly easily. The book offers a clear explanation of exactly what factors you control in ASM programming.	Franklin Watts
The Programming Guide to the Z80 Chip	Robinson, Phillip R.			TAB Books cop.
Learning Assembly Language: A Guide for BASIC Programmers	Jackson, Hugo T. and Fischer, A.M.		The title itself should draw TI-BASIC programmers. Not about the Z80, but about a fictional chip called "GRAPE-80" which handles generalized instructions.	Harper and Row

Books in German

Title	Author	Year	Comment	Publisher
Z80 - 1979-80 Buch 3 : Interface-Technik and Anwendung	Nichols, Elizabeth A.	1980		Elektor Verlag
Z80 - 1979-80 Buch 1 : Einfuerung und programmierung	Nichols, Elizabeth A.	1979		Elektor Verlag
Z80 Applikationsbuch : Einfuerung in die programmierung und Interfacetechniken des Mikroprozessor Z80	klein, Michael	1983		Franzis
Mikrocomputer selbstgebaut und programmiert : Vom bauelement zum fertigen Z80-Computer	Klein, Rolf-Dieter	1984		Franzis
Mikrocomputer : Hard- und softwarepraxis : Anhand ausfuehrlicher Beisoiele und oroesserer Programme wird das Programmieren immer perfekter	Klein, Rolf-Dieter	1981		Franzis Verlag

Books in Danish/Swedish

Title	Author	Year	Comment	Publisher
Mikroprocessor : arkitektur, instruktioner og adresseringsmetoder	Gravsbjerg, Jens	1986		Teknisk Forlag
Maskinkod och assembler	unknown	1984		Bromberos
Maskinkode med amstrad	Lorentzen, Jørn	1985		Borgen
Z80 programmering	Tootill, Alan	1985		Teknisk Forlag
Z80 assembler programmering	Skræp, Mogens	1983+1985		M.S. Databøger
Maskinkode programmering med Z80	Skavin, Jesper	1985		Borgen
Programmera Z80	Zaks, Radnav	1984		Sybex

This list was optained from my local library and the Zshell FAQ, so it is probably not any where near complete. If you have any information besides the above please mail ME. Some of the comments was taken from the Zshell FAQ, maintained by Henrik Edlund

Thanks to: