1) A Widow, a Chihuahua, and Harry Truman (2001) by Mary Beth Crain.
2) German book Chihuahuas (2007) by Karin Biala-Gaub.
3) Chihuahua: Nuevas guias perros de raz (2007) by Ann Hearn, Spanish edition of a book originally published in English.
4) The Complete Chihuahua (revised fourth edition, 1967) multi-authored reference by James Watson, Anna B. Vinyard, Rosina Casselli, Milo G. Denlinger, and the Rev. Russell E. Kauffman. The first edition was in 1947.