1) Chihuahua: Your Happy Healthy Pet (2nd edition, 2006) by Elaine Waldorf Gewirtz.
2) Chi Chi in Cyberspace (1999) by Karren Lee Silver and Marc Phillips, a children's book that teaches very young children motorskills and computer skills.
3) The Chihuahua (1988) by Anna Katherine Nicholas, one of many rather generic petshop-distributed books on chihuahuas from TFH Publications. It is one of the best of the introductory type books because it has 129 color photos and an excellent section that discusses significant pedigree lines: Flints, Bayards, Dartons, Pratts, etc. Dog owners with chis from these famous lines will enjoy seeing their own pet's ancestors.
4) The Chihuahua: A Complete Presentation, With Illustrations, of the Origin, Development, Breeding, Showing, Training, Housing, Care, Feeding and General Management, of this his Unique Toy Breed of Dog (1959) by Rev. Russell Emerson Kauffman. This is the second edition of The Chihuahua: Blue Ribbon Books on Dogs (1952). Some believe "Rev. Kauffman" was the pen name of Ida H. Garrett, on what basis I do not know. Garrett wrote a pamphlet The Chihuahua (1933, reprinted 2000) backstitched in plain blue cardstock. Rev. Kauffman also wrote When the Muses Get Workin' and Other Poems (1932) and I see no reason to doubt his existence.