Johnson's text (Acoustic and Auditory Phonetics) is required. Several chapters from the text by Ladefoged and Johnson (A Course in Phonetics) are also required, however earlier editions of this text are available and are perfectly appropriate for this course. (I have a few copies that I can loan to students.) You might want to attend class before purchasing this rather expensive text.
Acoustic and auditory phonetics, Author: Keith Johnson., Publisher: Wiley-Blackwell 3rd ed.
A course in phonetics, Author: Peter Ladefoged, Keith Johnson., Publisher: Wadsworth/Cengage Learning 6th ed.
Other Textbook Editions OK.