1) Help Find Honey! (2006) by Ben M. Baglio, children's fiction about a lost chihuahua, in the Pet Finders Club series.
2) Lucia Gasparini and Ginevra Gasparini's Il Grande Libro del Chihuahua (2009), published in Italy.
3) The Word's Smallest Dogs (1996) by Barbara J. Patten and John M. Patten, Jr. Although the cover features a pug, the book covers other small breeds, so of course the chihuahua, as well as the Japanese chin, Yorkshire and silky terriers, and the pomeranian.
4) Los Chihuahuas (1989) by Myrle Hale. Seventyi-two page booklet for chihuahua breeders of show dogs, self-published by Mrs. Myrle Hale of Dover, Florida.