Left) All About Toy Dogs (1965) by Viva Leone Ricketts.
Covers a great many miniature breeds besides
chihuahuas and the xoloitzcuintli (Mexican hairless).
Right) The Complete Book of Toy Dogs (1969)
by Cecil Gordon Eugene Wimhurst, covering several
small breeds, including of course the chihuahua.
Left) Grooming and Showing Toy Dogs (1976)
by Peggy A. Hogg and Robert J. Berndt.
Covers many small breeds inclusive of chihuahua.
Right) Living with Small and Toy Dogs:
Training, Behavior, and Personality Differences (1996)
by Terry Jester. Overview of toy breeds, including chihuahuas.